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{{Automatic taxobox
{{Automatic taxobox
| name = Birds
| name = Birds
| fossil_range = <br />{{fossilrange|72|0|[[Late Cretaceous]] – [[Holocene|present]], 72–0 [[Year#SI prefix multipliers|Ma]]| | earliest=115.5|PS= |ref=<ref name="Field2020">{{Cite journal|title=Late Cretaceous neornithine from Europe illuminates the origins of crown birds|last1=Field|first1=Daniel J.|last2=Benito|first2=Juan|date=March 2020|journal=Nature|issue=7799|doi=10.1038/s41586-020-2096-0|volume=579|pages=397–401|issn=0028-0836|last3=Chen|first3=Albert|last4=Jagt|first4=John W. M.|last5=Ksepka|first5=Daniel T.|pmid=32188952|bibcode=2020Natur.579..397F|s2cid=212937591|url=https://www.repository.cam.ac.uk/handle/1810/303639}}</ref><ref name="DePietriWilaru">{{cite journal |last1=De Pietri |first1=Vanesa L. |last2=Scofield |first2=R. Paul |last3=Zelenkov |first3=Nikita |last4=Boles |first4=Walter E. |last5=Worthy |first5=Trevor H. |title=The unexpected survival of an ancient lineage of anseriform birds into the Neogene of Australia: the youngest record of Presbyornithidae |journal=Royal Society Open Science |date=February 2016 |volume=3 |issue=2 |pages=150635 |doi=10.1098/rsos.150635|pmid=26998335 |pmc=4785986 |bibcode=2016RSOS....350635D |doi-access=free }}</ref>}}<small>Possible [[Early Cretaceous]] or early Late Cretaceous origin based on [[molecular clock]]<ref name="Yonezawa2017">{{cite journal |vauthors=Yonezawa, T, ''et al''. |date=2017 |title=Phylogenomics and Morphology of Extinct Paleognaths Reveal the Origin and Evolution of the Ratites |journal=Current Biology |volume=27 |number=1 |pages=68–77 |doi=10.1016/j.cub.2016.10.029|pmid=27989673 |doi-access=free }}</ref><ref name="kuhl2020">{{cite journal |first1=H |last1=Kuhl |first2=C |last2=Frankl-Vilches |first3=A |last3=Bakker |first4=G |last4=Mayr |first5=G |last5=Nikolaus |first6=S T |last6=Boerno |first7=S |last7=Klages |first8=B |last8=Timmermann |first9=M |last9=Gahr |date=2020 |volume=38 |number=1 |pages=108–127 |url=https://academic.oup.com/mbe/advance-article/doi/10.1093/molbev/msaa191/5891114 |title=An unbiased molecular approach using 3'UTRs resolves the avian family-level tree of life |journal=Molecular Biology and Evolution |doi=10.1093/molbev/msaa191|pmid=32781465 |pmc=7783168 |hdl=21.11116/0000-0007-B72A-C |hdl-access=free }}</ref></small>
| fossil_range = <br />{{fossilrange|72|0|[[Late Cretaceous]] – [[Holocene|present]], 72–0 [[Year#SI prefix multipliers|Ma]]| | earliest=115.5|PS= |ref=<ref name="Field2020">{{Cite journal|title=Late Cretaceous neornithine from Europe illuminates the origins of crown birds|last1=Field|first1=Daniel J.|last2=Benito|first2=Juan|date=March 2020|journal=Nature|issue=7799|doi=10.1038/s41586-020-2096-0|volume=579|pages=397–401|issn=0028-0836|last3=Chen|first3=Albert|last4=Jagt|first4=John W. M.|last5=Ksepka|first5=Daniel T.|pmid=32188952|bibcode=2020Natur.579..397F|s2cid=212937591|url=https://www.repository.cam.ac.uk/handle/1810/303639}}</ref><ref name="DePietriWilaru">{{cite journal |last1=De Pietri |first1=Vanesa L. |last2=Scofield |first2=R. Paul |last3=Zelenkov |first3=Nikita |last4=Boles |first4=Walter E. |last5=Worthy |first5=Trevor H. |title=The unexpected survival of an ancient lineage of anseriform birds into the Neogene of Australia: the youngest record of Presbyornithidae |journal=Royal Society Open Science |date=February 2016 |volume=3 |issue=2 |pages=150635 |doi=10.1098/rsos.150635|pmid=26998335 |pmc=4785986 |bibcode=2016RSOS....350635D |doi-access=free }}</ref>}}<small>Possible [[Early Cretaceous]] or early Late Cretaceous origin based on [[molecular clock]]<ref name="Yonezawa2017">{{cite journal |vauthors=Yonezawa, T, ''et al''. |date=2017 |title=Phylogenomics and Morphology of Extinct Paleognaths Reveal the Origin and Evolution of the Ratites |journal=Current Biology |volume=27 |number=1 |pages=68–77 |doi=10.1016/j.cub.2016.10.029|pmid=27989673 |doi-access=free }}</ref><ref name="kuhl2020">{{cite journal |first1=H |last1=Kuhl |first2=C |last2=Frankl-Vilches |first3=A |last3=Bakker |first4=G |last4=Mayr |first5=G |last5=Nikolaus |first6=S T |last6=Boerno |first7=S |last7=Klages |first8=B |last8=Timmermann |first9=M |last9=Gahr |date=2020 |volume=38 |number=1 |pages=108–127 |url=https://academic.oup.com/mbe/advance-article/doi/10.1093/molbev/msaa191/5891114 |title=An unbiased molecular approach using 3'UTRs resolves the avian family-level tree of life |journal=Molecular Biology and Evolution |doi=10.1093/molbev/msaa191|pmid=32781465 |pmc=7783168 |hdl=21.11116/0000-0007-B72A-C |hdl-access=free }}</ref><ref name="crouch2022">Crouch, N.M.A. (2022) Interpreting the fossil record and the origination of birds. ''bioRxiv'', doi: https://doi.org/10.1101/2022.05.19.492716</ref></small>
| image = <center><imagemap>
| image = <center><imagemap>
File:Bird Diversity 2013.png|300px
File:Bird Diversity 2013.png|300px
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| authority = [[Carl Linnaeus|Linnaeus]], [[10th edition of Systema Naturae|1758]]<ref>{{cite web| url= http://taxonomicon.taxonomy.nl/TaxonTree.aspx?id=80129&tree=0.1| title=Systema Naturae 2000 / Classification, Class Aves | access-date=11 June 2012 | last=Brands | first=Sheila | date=14 August 2008 | work=Project: The Taxonomicon }}</ref>
| authority = [[Carl Linnaeus|Linnaeus]], [[10th edition of Systema Naturae|1758]]<ref>{{cite web| url= http://taxonomicon.taxonomy.nl/TaxonTree.aspx?id=80129&tree=0.1| title=Systema Naturae 2000 / Classification, Class Aves | access-date=11 June 2012 | last=Brands | first=Sheila | date=14 August 2008 | work=Project: The Taxonomicon }}</ref>
| subdivision_ranks = Extant [[clade]]s
| subdivision_ranks = Extant [[clade]]s
| subdivision = *[[Palaeognathae]]
| subdivision = *[[Palaeognathae]] (ratites and tinamou)
*[[Neognathae]]
*[[Neognathae]]
**[[Pangalloanserae]]
**[[Pangalloanserae]] (fowl)
**[[Neoaves]]
**[[Neoaves]]
| synonyms = * Neornithes <small>Gadow, 1883</small>
| synonyms = * Neornithes <small>Gadow, 1883</small>
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'''Birds''' are a group of [[warm-blooded]] [[vertebrate]]s constituting the [[class (biology)|class]] '''Aves''' {{IPAc-en|ˈ|eɪ|v|iː|z}}, characterised by [[feather]]s, toothless beaked jaws, the [[Oviparity|laying]] of [[Eggshell|hard-shelled]] eggs, a high [[Metabolism|metabolic]] rate, a four-chambered [[heart]], and a strong yet lightweight [[Bird skeleton|skeleton]]. Birds live worldwide and range in size from the {{convert|5.5|cm|in|sigfig=2|abbr=on}} [[bee hummingbird]] to the {{convert|2.8|m|ftin|sigfig=2|abbr=on}} [[Common ostrich|ostrich]]. There are about ten thousand living species, more than half of which are [[passerine]], or "perching" birds. Birds have {{Birdgloss|wings}} whose development varies according to species; the only known groups without wings are the extinct [[moa]] and [[elephant bird]]s. Wings, which evolved from [[forelimb]]s, gave birds the ability to fly, although further evolution has led to the [[Flightless bird|loss of flight in some birds]], including [[ratite]]s, [[penguin]]s, and diverse [[endemism|endemic]] island species. The digestive and respiratory systems of birds are also uniquely adapted for flight. Some bird species of aquatic environments, particularly [[seabird]]s and some [[Water bird|waterbirds]], have further evolved for swimming.
'''Birds''' are a group of [[warm-blooded]] [[vertebrate]]s constituting the [[class (biology)|class]] '''Aves''' {{IPAc-en|ˈ|eɪ|v|iː|z}}, characterised by [[feather]]s, toothless beaked jaws, the [[Oviparity|laying]] of [[Eggshell|hard-shelled]] eggs, a high [[Metabolism|metabolic]] rate, a four-chambered [[heart]], and a strong yet lightweight [[Bird skeleton|skeleton]]. Birds live worldwide and range in size from the {{convert|5.5|cm|in|sigfig=2|abbr=on}} [[bee hummingbird]] to the {{convert|2.8|m|ftin|sigfig=2|abbr=on}} [[Common ostrich|ostrich]]. There are about ten thousand living species, more than half of which are [[passerine]], or "perching" birds. Birds have {{Birdgloss|wings}} whose development varies according to species; the only known groups without wings are the extinct [[moa]] and [[elephant bird]]s. Wings, which evolved from [[forelimb]]s, gave birds the ability to fly, although further evolution has led to the [[Flightless bird|loss of flight in some birds]], including [[ratite]]s, [[penguin]]s, and diverse [[endemism|endemic]] island species. The digestive and respiratory systems of birds are also uniquely adapted for flight. Some bird species of aquatic environments, particularly [[seabird]]s and some [[Water bird|waterbirds]], have further evolved for swimming.


Birds are [[feathered dinosaur|feathered]] [[theropod]] [[dinosaur]]s and constitute the [[Origin of birds|only known living dinosaurs]]. Likewise, birds are considered [[reptile]]s in the modern [[Cladistics|cladistic]] sense of the term, and their closest living relatives are the [[crocodilia]]ns. Birds are descendants of the primitive [[Avialae|avialans]] (whose members include ''[[Archaeopteryx]]'') which first appeared about 160&nbsp;million years ago (mya) in China. According to DNA evidence, modern birds ('''Neornithes''') evolved in the [[Middle Cretaceous|Middle]] to [[Late Cretaceous]], and diversified dramatically around the time of the [[Cretaceous–Paleogene extinction event]] 66 mya, which killed off the [[pterosaur]]s and all non-avian dinosaurs.
Birds are [[feathered dinosaur|feathered]] [[theropod]] [[dinosaur]]s and constitute the [[Origin of birds|only known living dinosaurs]]. Likewise, birds are considered [[reptile]]s in the modern [[Cladistics|cladistic]] sense of the term, and their closest living relatives are the [[crocodilia]]ns. Birds are descendants of the primitive [[Avialae|avialans]] (whose members include ''[[Archaeopteryx]]'') which first appeared about 160&nbsp;million years ago (mya) in China. According to DNA evidence, modern birds ('''Neornithes''') evolved in the [[Middle Cretaceous|Middle]] to [[Late Cretaceous]], and diversified dramatically around the time of the [[Cretaceous–Paleogene extinction event]] 66 mya, which killed off the [[pterosaur]]s and all non-avian dinosaurs.<ref name="crouch2022"/>


Many [[social animal|social species]] pass on knowledge across generations, which is considered [[Animal culture#Examples of culturally transmitted behaviors in birds|a form of culture]]. Birds are social, communicating with visual signals, calls, and [[bird vocalization|songs]], and participating in such behaviours as [[helpers at the nest|cooperative breeding]] and hunting, [[Flocking (behavior)|flocking]], and [[Mobbing (animal behavior)|mobbing]] of predators. The vast majority of bird species are socially (but not necessarily sexually) [[Monogamy in animals|monogamous]], usually for one breeding season at a time, sometimes for years, and rarely for life. Other species have breeding systems that are [[Polygyny in animals|polygynous]] (one male with many females) or, rarely, [[Polyandry in nature|polyandrous]] (one female with many males). Birds produce offspring by laying eggs which are fertilised through [[sexual reproduction]]. They are usually laid in a nest and [[Avian incubation|incubated]] by the parents. Most birds have an extended period of parental care after hatching.
Many [[social animal|social species]] pass on knowledge across generations, which is considered [[Animal culture#Examples of culturally transmitted behaviors in birds|a form of culture]]. Birds are social, communicating with visual signals, calls, and [[bird vocalization|songs]], and participating in such behaviours as [[helpers at the nest|cooperative breeding]] and hunting, [[Flocking (behavior)|flocking]], and [[Mobbing (animal behavior)|mobbing]] of predators. The vast majority of bird species are socially (but not necessarily sexually) [[Monogamy in animals|monogamous]], usually for one breeding season at a time, sometimes for years, and rarely for life. Other species have breeding systems that are [[Polygyny in animals|polygynous]] (one male with many females) or, rarely, [[Polyandry in nature|polyandrous]] (one female with many males). Birds produce offspring by laying eggs which are fertilised through [[sexual reproduction]]. They are usually laid in a nest and [[Avian incubation|incubated]] by the parents. Most birds have an extended period of parental care after hatching.
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===Definition===
===Definition===
Aves and a sister group, the order [[Crocodilia]], contain the only living representatives of the reptile clade [[Archosauria]]. During the late 1990s, Aves was most commonly defined [[Phylogenetics|phylogenetically]] as all descendants of the [[most recent common ancestor]] of modern birds and ''[[Archaeopteryx|Archaeopteryx lithographica]]''.<ref>{{Cite book|last=Padian|first=Kevin|author-link=Kevin Padian|author2=L.M. Chiappe |author3=Chiappe LM |editor=[[Philip J. Currie]] |editor2=Kevin Padian |title=Encyclopedia of Dinosaurs|year=1997|publisher=[[Academic Press]]|location=San Diego|pages=41–96|chapter=Bird Origins|isbn=0-12-226810-5}}</ref> However, an earlier definition proposed by [[Jacques Gauthier]] gained wide currency in the 21st century, and is used by many scientists including adherents of the [[Phylocode]] system. Gauthier defined Aves to include only the [[crown group]] of the set of modern birds. This was done by excluding most groups known only from [[fossils]], and assigning them, instead, to the broader group Avialae,<ref>{{Cite book|last=Gauthier |first=Jacques|editor=Kevin Padian |title=The Origin of Birds and the Evolution of Flight|series= Memoirs of the California Academy of Science '''8'''|year=1986|pages=1–55|chapter=Saurischian Monophyly and the origin of birds|isbn=0-940228-14-9 |publisher=Published by California Academy of Sciences |location=San Francisco, CA}}</ref> in part to avoid the uncertainties about the placement of ''Archaeopteryx'' in relation to animals traditionally thought of as theropod dinosaurs.{{citation needed|date=May 2022}} <!-- See WP:RS<ref>http://www.phylonames.org/forum/viewtopic.php?t=7 {{Dead link|date=February 2022}}</ref> --><!--Mayr et al. 2005 "A well-preserved Archaeopteryx specimen with theropod features" + comment + Mayr's comment on the comment-->
Aves and a sister group, the order [[Crocodilia]], contain the only living representatives of the reptile clade [[Archosauria]]. During the late 1990s, Aves was most commonly defined [[Phylogenetics|phylogenetically]] as all descendants of the [[most recent common ancestor]] of modern birds and ''[[Archaeopteryx|Archaeopteryx lithographica]]''.<ref>{{Cite book|last=Padian|first=Kevin|author-link=Kevin Padian|author2=L.M. Chiappe |author3=Chiappe LM |editor=[[Philip J. Currie]] |editor2=Kevin Padian |title=Encyclopedia of Dinosaurs|year=1997|publisher=[[Academic Press]]|location=San Diego|pages=41–96|chapter=Bird Origins|isbn=0-12-226810-5}}</ref> However, an earlier definition proposed by [[Jacques Gauthier]] gained wide currency in the 21st century, and is used by many scientists including adherents to the ''[[PhyloCode]]''. Gauthier defined Aves to include only the [[crown group]] of the set of modern birds. This was done by excluding most groups known only from [[fossils]], and assigning them, instead, to the broader group Avialae,<ref>{{Cite book|last=Gauthier |first=Jacques|editor=Kevin Padian |title=The Origin of Birds and the Evolution of Flight|series= Memoirs of the California Academy of Science '''8'''|year=1986|pages=1–55|chapter=Saurischian Monophyly and the origin of birds|isbn=0-940228-14-9 |publisher=Published by California Academy of Sciences |location=San Francisco, CA}}</ref> in part to avoid the uncertainties about the placement of ''Archaeopteryx'' in relation to animals traditionally thought of as theropod dinosaurs.{{citation needed|date=May 2022}} <!-- See WP:RS<ref>http://www.phylonames.org/forum/viewtopic.php?t=7 {{Dead link|date=February 2022}}</ref> --><!--Mayr et al. 2005 "A well-preserved Archaeopteryx specimen with theropod features" + comment + Mayr's comment on the comment-->


Gauthier and de Queiroz<ref name="gauthier&dequeiroz2001"/> identified four different definitions for the same biological name "Aves", which is a problem. The authors proposed to reserve the term Aves only for the crown group consisting of the last common ancestor of all living birds and all of its descendants, which corresponds to meaning number 4 below. He assigned other names to the other groups.{{citation needed|date=May 2022}}
Gauthier and de Queiroz<ref name="gauthier&dequeiroz2001"/> identified four different definitions for the same biological name "Aves", which is a problem. The authors proposed to reserve the term Aves only for the crown group consisting of the last common ancestor of all living birds and all of its descendants, which corresponds to meaning number 4 below. He assigned other names to the other groups.{{citation needed|date=May 2022}}
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Many species of the second major avialan lineage to diversify, the [[Euornithes]] (meaning "true birds", because they include the ancestors of modern birds), were semi-aquatic and specialised in eating fish and other small aquatic organisms. Unlike the Enantiornithes, which dominated land-based and arboreal habitats, most early euornithes lacked [[Perching bird|perching]] adaptations and seem to have included shorebird-like species, waders, and swimming and diving species.{{citation needed|date=May 2022}}
Many species of the second major avialan lineage to diversify, the [[Euornithes]] (meaning "true birds", because they include the ancestors of modern birds), were semi-aquatic and specialised in eating fish and other small aquatic organisms. Unlike the Enantiornithes, which dominated land-based and arboreal habitats, most early euornithes lacked [[Perching bird|perching]] adaptations and seem to have included shorebird-like species, waders, and swimming and diving species.{{citation needed|date=May 2022}}


The latter included the superficially [[gull]]-like ''[[Ichthyornis]]''<ref>{{Cite journal |last=Clarke |first=Julia A. |year=2004 |title=Morphology, Phylogenetic Taxonomy, and Systematics of ''Ichthyornis'' and ''Apatornis'' (Avialae: Ornithurae) |journal=Bulletin of the American Museum of Natural History |volume=286 |pages=1–179 |doi=10.1206/0003-0090(2004)286<0001:MPTASO>2.0.CO;2 |url=http://digitallibrary.amnh.org/dspace/bitstream/2246/454/1/B286.pdf |hdl=2246/454 |access-date=14 September 2007 |archive-date=3 March 2009 |archive-url=https://web.archive.org/web/20090303221206/http://digitallibrary.amnh.org/dspace/bitstream/2246/454/1/B286.pdf |url-status=dead }}</ref> and the [[Hesperornithiformes]], which became so well adapted to hunting fish in marine environments that they lost the ability to fly and became primarily aquatic.<ref name="chiappe2007"/> The early euornithes also saw the development of many traits associated with modern birds, like strongly keeled breastbones, toothless, beaked portions of their jaws (though most non-avian euornithes retained teeth in other parts of the jaws).<ref name=louchart2011>{{cite journal | last1 = Louchart | first1 = A. | last2 = Viriot | first2 = L. | year = 2011 | title = From snout to beak: the loss of teeth in birds | url = http://198.81.200.84/trends/ecology-evolution/abstract/S0169-5347%2811%2900264-3?switch=standard | journal = Trends in Ecology & Evolution | volume = 26 | issue = 12 | pages = 663–673 | doi = 10.1016/j.tree.2011.09.004 | pmid = 21978465 | url-status = dead | archive-url = https://web.archive.org/web/20140728053547/http://198.81.200.84/trends/ecology-evolution/abstract/S0169-5347%2811%2900264-3?switch=standard | archive-date = 28 July 2014}}</ref> Euornithes also included the first avialans to develop true [[pygostyle]] and a fully mobile fan of tail feathers,<ref name=yixianornis>{{cite journal | last1 = Clarke | first1 = J.A. | last2 = Zhou | first2 = Z. | last3 = Zhang | first3 = F. | date = March 2006 | title = Insight into the evolution of avian flight from a new clade of Early Cretaceous ornithurines from China and the morphology of ''Yixianornis grabaui'' | journal = Journal of Anatomy | volume = 208 | issue = 3| pages = 287–308 | doi=10.1111/j.1469-7580.2006.00534.x | pmid=16533313 | pmc=2100246}}</ref> which may have replaced the "hind wing" as the primary mode of aerial maneuverability and braking in flight.<ref name=zhengetal2013/>
The latter included the superficially [[gull]]-like ''[[Ichthyornis]]''<ref>{{Cite journal |last=Clarke |first=Julia A. |year=2004 |title=Morphology, Phylogenetic Taxonomy, and Systematics of ''Ichthyornis'' and ''Apatornis'' (Avialae: Ornithurae) |journal=Bulletin of the American Museum of Natural History |volume=286 |pages=1–179 |doi=10.1206/0003-0090(2004)286<0001:MPTASO>2.0.CO;2 |url=http://digitallibrary.amnh.org/dspace/bitstream/2246/454/1/B286.pdf |hdl=2246/454 |s2cid=84035285 |access-date=14 September 2007 |archive-date=3 March 2009 |archive-url=https://web.archive.org/web/20090303221206/http://digitallibrary.amnh.org/dspace/bitstream/2246/454/1/B286.pdf |url-status=dead }}</ref> and the [[Hesperornithiformes]], which became so well adapted to hunting fish in marine environments that they lost the ability to fly and became primarily aquatic.<ref name="chiappe2007"/> The early euornithes also saw the development of many traits associated with modern birds, like strongly keeled breastbones, toothless, beaked portions of their jaws (though most non-avian euornithes retained teeth in other parts of the jaws).<ref name=louchart2011>{{cite journal | last1 = Louchart | first1 = A. | last2 = Viriot | first2 = L. | year = 2011 | title = From snout to beak: the loss of teeth in birds | url = http://198.81.200.84/trends/ecology-evolution/abstract/S0169-5347%2811%2900264-3?switch=standard | journal = Trends in Ecology & Evolution | volume = 26 | issue = 12 | pages = 663–673 | doi = 10.1016/j.tree.2011.09.004 | pmid = 21978465 | url-status = dead | archive-url = https://web.archive.org/web/20140728053547/http://198.81.200.84/trends/ecology-evolution/abstract/S0169-5347%2811%2900264-3?switch=standard | archive-date = 28 July 2014}}</ref> Euornithes also included the first avialans to develop true [[pygostyle]] and a fully mobile fan of tail feathers,<ref name=yixianornis>{{cite journal | last1 = Clarke | first1 = J.A. | last2 = Zhou | first2 = Z. | last3 = Zhang | first3 = F. | date = March 2006 | title = Insight into the evolution of avian flight from a new clade of Early Cretaceous ornithurines from China and the morphology of ''Yixianornis grabaui'' | journal = Journal of Anatomy | volume = 208 | issue = 3| pages = 287–308 | doi=10.1111/j.1469-7580.2006.00534.x | pmid=16533313 | pmc=2100246}}</ref> which may have replaced the "hind wing" as the primary mode of aerial maneuverability and braking in flight.<ref name=zhengetal2013/>


A study on [[mosaic evolution]] in the avian skull found that the [[Most recent common ancestor|last common ancestor]] of all Neornithes might have had a beak similar to that of the modern [[hook-billed vanga]] and a skull similar to that of the [[Eurasian golden oriole]]. As both species are small aerial and canopy foraging omnivores, a similar ecological niche was inferred for this hypothetical ancestor.<ref>{{cite journal | last1 = Felice | first1 = Ryan N. | last2 = Goswami | first2 = Anjali | year = 2018 | title = Developmental origins of mosaic evolution in the avian cranium | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 115 | issue = 3| pages = 555–60 | doi = 10.1073/pnas.1716437115 | pmid = 29279399 | pmc = 5776993 | doi-access = free }}</ref>
A study on [[mosaic evolution]] in the avian skull found that the [[Most recent common ancestor|last common ancestor]] of all Neornithes might have had a beak similar to that of the modern [[hook-billed vanga]] and a skull similar to that of the [[Eurasian golden oriole]]. As both species are small aerial and canopy foraging omnivores, a similar ecological niche was inferred for this hypothetical ancestor.<ref>{{cite journal | last1 = Felice | first1 = Ryan N. | last2 = Goswami | first2 = Anjali | year = 2018 | title = Developmental origins of mosaic evolution in the avian cranium | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 115 | issue = 3| pages = 555–60 | doi = 10.1073/pnas.1716437115 | pmid = 29279399 | pmc = 5776993 | doi-access = free }}</ref>
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</ref> Depending on the [[alpha taxonomy|taxonomic]] viewpoint, the number of known living bird species varies anywhere from 9,800<ref>{{Cite book|title=The Clements Checklist of Birds of the World |first=James F. |last=Clements |edition=6th |author-link=James Clements |location=Ithaca |publisher=[[Cornell University Press]] |year=2007 |isbn=978-0-8014-4501-9|title-link=The Clements Checklist of Birds of the World }}</ref> to 10,758.<ref>{{Cite journal|doi=10.14344/ioc.ml.9.2|title=Welcome |journal=IOC World Bird List 9.2}}</ref>
</ref> Depending on the [[alpha taxonomy|taxonomic]] viewpoint, the number of known living bird species varies anywhere from 9,800<ref>{{Cite book|title=The Clements Checklist of Birds of the World |first=James F. |last=Clements |edition=6th |author-link=James Clements |location=Ithaca |publisher=[[Cornell University Press]] |year=2007 |isbn=978-0-8014-4501-9|title-link=The Clements Checklist of Birds of the World }}</ref> to 10,758.<ref>{{Cite journal|doi=10.14344/ioc.ml.9.2|title=Welcome |journal=IOC World Bird List 9.2}}</ref>


The discovery of ''[[Vegavis]]'' from the [[Maastrichtian]], the last stage of the Late Cretaceous proved that the diversification of modern birds started before the [[Cenozoic]] era.<ref>{{cite journal |last1=Clarke |first1=Julia A. |last2=Tambussi |first2=Claudia P. |last3=Noriega |first3=Jorge I. |last4=Erickson |first4=Gregory M. |last5=Ketcham |first5=Richard A. |title=Definitive fossil evidence for the extant avian radiation in the Cretaceous |journal=Nature |date=January 2005 |volume=433 |issue=7023 |pages=305–308 |doi=10.1038/nature03150 |pmid=15662422 |bibcode=2005Natur.433..305C |s2cid=4354309 }}</ref> The affinities of an earlier fossil, the possible [[Galliformes|galliform]] ''[[Austinornis]] lentus'', dated to about 85&nbsp;million years ago,<ref>{{cite journal | last1 = Clarke | first1 = J.A. | year = 2004 | title = Morphology, phylogenetic taxonomy, and systematics of ''Ichthyornis'' and ''Apatornis'' (Avialae: Ornithurae) | url = http://digitallibrary.amnh.org/dspace/bitstream/handle/2246/454/B286.?sequence=1 | journal = Bulletin of the American Museum of Natural History | volume = 286 | pages = 1–179 | doi = 10.1206/0003-0090(2004)286<0001:mptaso>2.0.co;2 | hdl = 2246/454 | access-date = 22 March 2015 | archive-date = 19 June 2015 | archive-url = https://web.archive.org/web/20150619214015/http://digitallibrary.amnh.org/dspace/bitstream/handle/2246/454/B286.?sequence=1 | url-status = dead }}</ref> are still too controversial to provide a fossil evidence of modern bird diversification. In 2020, ''[[Asteriornis]]'' from the Maastrichtian was described, it appears to be a close relative of [[Galloanserae]], the earliest diverging lineage within Neognathae.<ref name="Field2020"/>
The discovery of ''[[Vegavis]]'' from the [[Maastrichtian]], the last stage of the Late Cretaceous proved that the diversification of modern birds started before the [[Cenozoic]] era.<ref>{{cite journal |last1=Clarke |first1=Julia A. |last2=Tambussi |first2=Claudia P. |last3=Noriega |first3=Jorge I. |last4=Erickson |first4=Gregory M. |last5=Ketcham |first5=Richard A. |title=Definitive fossil evidence for the extant avian radiation in the Cretaceous |journal=Nature |date=January 2005 |volume=433 |issue=7023 |pages=305–308 |doi=10.1038/nature03150 |pmid=15662422 |bibcode=2005Natur.433..305C |s2cid=4354309 }}</ref> The affinities of an earlier fossil, the possible [[Galliformes|galliform]] ''[[Austinornis]] lentus'', dated to about 85&nbsp;million years ago,<ref>{{cite journal | last1 = Clarke | first1 = J.A. | year = 2004 | title = Morphology, phylogenetic taxonomy, and systematics of ''Ichthyornis'' and ''Apatornis'' (Avialae: Ornithurae) | url = http://digitallibrary.amnh.org/dspace/bitstream/handle/2246/454/B286.?sequence=1 | journal = Bulletin of the American Museum of Natural History | volume = 286 | pages = 1–179 | doi = 10.1206/0003-0090(2004)286<0001:mptaso>2.0.co;2 | hdl = 2246/454 | s2cid = 84035285 | access-date = 22 March 2015 | archive-date = 19 June 2015 | archive-url = https://web.archive.org/web/20150619214015/http://digitallibrary.amnh.org/dspace/bitstream/handle/2246/454/B286.?sequence=1 | url-status = dead }}</ref> are still too controversial to provide a fossil evidence of modern bird diversification. In 2020, ''[[Asteriornis]]'' from the Maastrichtian was described, it appears to be a close relative of [[Galloanserae]], the earliest diverging lineage within Neognathae.<ref name="Field2020"/>


Most studies agree on a Cretaceous age for the most recent common ancestor of modern birds but estimates range from the [[Early Cretaceous]]<ref name="Yonezawa2017"/><ref name=divergence>{{Cite journal | last1 = Lee | first1 = Michael SY | last2 = Cau | first2 = Andrea | last3 = Naish | first3 = Darren | last4 = Dyke | first4 = Gareth J. | date = May 2014 | title = Morphological Clocks in Paleontology, and a Mid-Cretaceous Origin of Crown Aves | journal = Systematic Biology | publisher = Oxford Journals | doi = 10.1093/sysbio/syt110 | volume=63 |issue=1 | pages=442–449 | pmid=24449041| url = https://academic.oup.com/sysbio/article-pdf/63/3/442/9164850/syt110.pdf | doi-access = free }}</ref> to the latest [[Late Cretaceous]].<ref name=Prum2015>{{cite journal | last1 = Prum | first1 = R.O. | display-authors = et al | year = 2015 | title = A comprehensive phylogeny of birds (Aves) using targeted next-generation DNA sequencing | journal = Nature | volume = 526 | issue = 7574 | pages = 569–573 | bibcode = 2015Natur.526..569P | doi = 10.1038/nature15697 | pmid = 26444237 | s2cid = 205246158 }}</ref><ref name="kuhl2020"/> Similarly, there is no agreement on whether most of the early diversification of modern birds occurred before or after the [[Cretaceous–Paleogene extinction event|Cretaceous–Palaeogene extinction event]].<ref name="Ericson">{{Cite journal |last1=Ericson |first1=Per G.P. |year=2006 |title=Diversification of Neoaves: integration of molecular sequence data and fossils |journal=[[Biology Letters]] |volume=2 |issue=4 |pages=543–547 |doi=10.1098/rsbl.2006.0523 |pmid=17148284 |url=http://www.senckenberg.de/files/content/forschung/abteilung/terrzool/ornithologie/neoaves.pdf |last2=Anderson |first2=CL |last3=Britton |first3=T |last4=Elzanowski |first4=A |last5=Johansson |first5=US |last6=Källersjö |first6=M |last7=Ohlson |first7=JI |last8=Parsons |first8=TJ |last9=Zuccon |first9=D |pmc=1834003 |first10=G. |last10=Mayr |display-authors=1 |access-date=4 July 2008 |archive-url=https://web.archive.org/web/20090325235703/http://www.senckenberg.de/files/content/forschung/abteilung/terrzool/ornithologie/neoaves.pdf |archive-date=25 March 2009 |url-status=dead }}</ref> This disagreement is in part caused by a divergence in the evidence; most molecular dating studies suggests a Cretaceous [[evolutionary radiation]], while fossil evidence points to a Cenozoic radiation (the so-called 'rocks' versus 'clocks' controversy). Previous attempts to reconcile molecular and fossil evidence have proved controversial,<ref name="Ericson"/><ref>{{Cite journal|last1=Brown |first1=Joseph W. |date=June 2007 |title=Nuclear DNA does not reconcile 'rocks' and 'clocks' in Neoaves: a comment on Ericson et al. |journal=[[Biology Letters]] |volume=3 |issue=3 |pages=257–259 |doi=10.1098/rsbl.2006.0611 |pmid=17389215 |last2=Payne |first2=RB |last3=Mindell |first3=DP |pmc=2464679}}</ref> but more recent estimates, using a more comprehensive sample of fossils and a new way of calibrating [[molecular clocks]], showed that while according to some studies, modern birds originated early in the Late Cretaceous in Western [[Gondwana]], a pulse of diversification in all major groups occurred around the Cretaceous–Palaeogene extinction event. Modern birds expanded from West Gondwana to the Laurasia through two routes. One route was an Antarctic interchange in the Paleogene. This can be confirmed with the presence of multiple avian groups in Australia and New Zealand. The other route was probably through North America, via land bridges, during the Paleocene. This allowed the expansion and diversification of Neornithes into the Holarctic and Paleotropics.<ref name=cracraft>{{cite journal |last1=Claramunt |first1=S. |last2=Cracraft |first2=J.|author-link2=Joel Cracraft |title=A new time tree reveals Earth history's imprint on the evolution of modern birds |journal=Sci Adv |date=2015 |volume=1 |issue=11 |doi=10.1126/sciadv.1501005 |pmc=4730849 |pmid=26824065 |page=e1501005|bibcode=2015SciA....1E1005C }}</ref> On the other hand, the occurrence of ''Asteriornis'' in the Northern Hemisphere challenges biogeographical hypotheses of a Gondwanan origin of crown birds.<ref name="Field2020"/>
Most studies agree on a Cretaceous age for the most recent common ancestor of modern birds but estimates range from the [[Early Cretaceous]]<ref name="Yonezawa2017"/><ref name=divergence>{{Cite journal | last1 = Lee | first1 = Michael SY | last2 = Cau | first2 = Andrea | last3 = Naish | first3 = Darren | last4 = Dyke | first4 = Gareth J. | date = May 2014 | title = Morphological Clocks in Paleontology, and a Mid-Cretaceous Origin of Crown Aves | journal = Systematic Biology | publisher = Oxford Journals | doi = 10.1093/sysbio/syt110 | volume=63 |issue=1 | pages=442–449 | pmid=24449041| url = https://academic.oup.com/sysbio/article-pdf/63/3/442/9164850/syt110.pdf | doi-access = free }}</ref> to the latest [[Late Cretaceous]].<ref name=Prum2015>{{cite journal | last1 = Prum | first1 = R.O. | display-authors = et al | year = 2015 | title = A comprehensive phylogeny of birds (Aves) using targeted next-generation DNA sequencing | journal = Nature | volume = 526 | issue = 7574 | pages = 569–573 | bibcode = 2015Natur.526..569P | doi = 10.1038/nature15697 | pmid = 26444237 | s2cid = 205246158 }}</ref><ref name="kuhl2020"/> Similarly, there is no agreement on whether most of the early diversification of modern birds occurred before or after the [[Cretaceous–Paleogene extinction event|Cretaceous–Palaeogene extinction event]].<ref name="Ericson">{{Cite journal |last1=Ericson |first1=Per G.P. |year=2006 |title=Diversification of Neoaves: integration of molecular sequence data and fossils |journal=[[Biology Letters]] |volume=2 |issue=4 |pages=543–547 |doi=10.1098/rsbl.2006.0523 |pmid=17148284 |url=http://www.senckenberg.de/files/content/forschung/abteilung/terrzool/ornithologie/neoaves.pdf |last2=Anderson |first2=CL |last3=Britton |first3=T |last4=Elzanowski |first4=A |last5=Johansson |first5=US |last6=Källersjö |first6=M |last7=Ohlson |first7=JI |last8=Parsons |first8=TJ |last9=Zuccon |first9=D |pmc=1834003 |first10=G. |last10=Mayr |display-authors=1 |access-date=4 July 2008 |archive-url=https://web.archive.org/web/20090325235703/http://www.senckenberg.de/files/content/forschung/abteilung/terrzool/ornithologie/neoaves.pdf |archive-date=25 March 2009 |url-status=dead }}</ref> This disagreement is in part caused by a divergence in the evidence; most molecular dating studies suggests a Cretaceous [[evolutionary radiation]], while fossil evidence points to a Cenozoic radiation (the so-called 'rocks' versus 'clocks' controversy). Previous attempts to reconcile molecular and fossil evidence have proved controversial,<ref name="Ericson"/><ref>{{Cite journal|last1=Brown |first1=Joseph W. |date=June 2007 |title=Nuclear DNA does not reconcile 'rocks' and 'clocks' in Neoaves: a comment on Ericson et al. |journal=[[Biology Letters]] |volume=3 |issue=3 |pages=257–259 |doi=10.1098/rsbl.2006.0611 |pmid=17389215 |last2=Payne |first2=RB |last3=Mindell |first3=DP |pmc=2464679}}</ref> but more recent estimates, using a more comprehensive sample of fossils and a new way of calibrating [[molecular clocks]], showed that while according to some studies, modern birds originated early in the Late Cretaceous in Western [[Gondwana]], a pulse of diversification in all major groups occurred around the Cretaceous–Palaeogene extinction event. Modern birds expanded from West Gondwana to the Laurasia through two routes. One route was an Antarctic interchange in the Paleogene. This can be confirmed with the presence of multiple avian groups in Australia and New Zealand. The other route was probably through North America, via land bridges, during the Paleocene. This allowed the expansion and diversification of Neornithes into the Holarctic and Paleotropics.<ref name=cracraft>{{cite journal |last1=Claramunt |first1=S. |last2=Cracraft |first2=J.|author-link2=Joel Cracraft |title=A new time tree reveals Earth history's imprint on the evolution of modern birds |journal=Sci Adv |date=2015 |volume=1 |issue=11 |doi=10.1126/sciadv.1501005 |pmc=4730849 |pmid=26824065 |page=e1501005|bibcode=2015SciA....1E1005C }}</ref> On the other hand, the occurrence of ''Asteriornis'' in the Northern Hemisphere challenges biogeographical hypotheses of a Gondwanan origin of crown birds.<ref name="Field2020"/>
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             |5=[[Opisthocomiformes]] (hoatzin)[[File:Cuvier-59-Hoazin huppé.jpg|60px]]
             |5=[[Opisthocomiformes]] (hoatzin)[[File:Cuvier-59-Hoazin huppé.jpg|60px]]
             |6=[[Strisores]] ([[Swift (bird)|swift]]s, [[hummingbird]]s, [[nightjar]]s and allies) [[File:Steatornis caripensis MHNT ZON STEA 1.jpg|60px]]
             |6=[[Strisores]] ([[Swift (bird)|swift]]s, [[hummingbird]]s, [[nightjar]]s and allies) [[File:Steatornis caripensis MHNT ZON STEA 1.jpg|60px]]
             |label7=[[Ardeae]]
             |label7=[[Phaethoquornithes]] |sublabel7=(Ardeae)
             |7={{clade
             |7={{clade
               |label1=[[Eurypygimorphae]]
               |label1=[[Eurypygimorphae]]
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===Genomics===
===Genomics===
{{See also|list of sequenced animal genomes}}
{{See also|list of sequenced animal genomes}}
{{as of|2010}}, the [[genome]] had been sequenced for only two birds, the [[chicken]] and the [[zebra finch]]. {{as of|2022}}  the genomes of 542 species of birds had been completed.  At least one genome has been sequenced from every order.<ref name="Holmes">{{cite journal |last1=Holmes |first1=Bob |title=Learning about birds from their genomes |journal=Knowable Magazine |date=10 February 2022 |doi=10.1146/knowable-021022-1 |url=https://knowablemagazine.org/article/living-world/2022/learning-about-birds-their-genomes  |doi-access=free |access-date=11 February 2022}}</ref><ref name="Bravo">{{cite journal |last1=Bravo |first1=Gustavo A. |last2=Schmitt |first2=C. Jonathan |last3=Edwards |first3=Scott V. |title=What Have We Learned from the First 500 Avian Genomes? |journal=Annual Review of Ecology, Evolution, and Systematics |date=3 November 2021 |volume=52 |issue=1 |pages=611–639 |doi=10.1146/annurev-ecolsys-012121-085928 |url=https://doi.org/10.1146/annurev-ecolsys-012121-085928 |access-date=11 February 2022 |issn=1543-592X}}</ref>  
{{as of|2010}}, the [[genome]] had been sequenced for only two birds, the [[chicken]] and the [[zebra finch]]. {{as of|2022}}  the genomes of 542 species of birds had been completed.  At least one genome has been sequenced from every order.<ref name="Holmes">{{cite journal |last1=Holmes |first1=Bob |title=Learning about birds from their genomes |journal=Knowable Magazine |date=10 February 2022 |doi=10.1146/knowable-021022-1 |url=https://knowablemagazine.org/article/living-world/2022/learning-about-birds-their-genomes  |doi-access=free |access-date=11 February 2022}}</ref><ref name="Bravo">{{cite journal |last1=Bravo |first1=Gustavo A. |last2=Schmitt |first2=C. Jonathan |last3=Edwards |first3=Scott V. |title=What Have We Learned from the First 500 Avian Genomes? |journal=Annual Review of Ecology, Evolution, and Systematics |date=3 November 2021 |volume=52 |issue=1 |pages=611–639 |doi=10.1146/annurev-ecolsys-012121-085928 |s2cid=239655248 |url=https://doi.org/10.1146/annurev-ecolsys-012121-085928 |access-date=11 February 2022 |issn=1543-592X}}</ref>  
These include at least one species in about 90% of extant avian families (218 out of 236 families recognised by the ''Howard and Moore Checklist'').<ref name="FengStiller2020">{{cite journal|last1=Feng|first1=Shaohong|last2=Stiller|first2=Josefin|last3=Deng|first3=Yuan|last4=Armstrong|first4=Joel|last5=Fang|first5=Qi|last6=Reeve|first6=Andrew Hart|last7=Xie|first7=Duo|last8=Chen|first8=Guangji|last9=Guo|first9=Chunxue|last10=Faircloth|first10=Brant C.|last11=Petersen|first11=Bent|last12=Wang|first12=Zongji|last13=Zhou|first13=Qi|last14=Diekhans|first14=Mark|last15=Chen|first15=Wanjun|last16=Andreu-Sánchez|first16=Sergio|last17=Margaryan|first17=Ashot|last18=Howard|first18=Jason Travis|last19=Parent|first19=Carole|last20=Pacheco|first20=George|last21=Sinding|first21=Mikkel-Holger S.|last22=Puetz|first22=Lara|last23=Cavill|first23=Emily|last24=Ribeiro|first24=Ângela M.|last25=Eckhart|first25=Leopold|last26=Fjeldså|first26=Jon|last27=Hosner|first27=Peter A.|last28=Brumfield|first28=Robb T.|last29=Christidis|first29=Les|last30=Bertelsen|first30=Mads F.|last31=Sicheritz-Ponten|first31=Thomas|last32=Tietze|first32=Dieter Thomas|last33=Robertson|first33=Bruce C.|last34=Song|first34=Gang|last35=Borgia|first35=Gerald|last36=Claramunt|first36=Santiago|last37=Lovette|first37=Irby J.|last38=Cowen|first38=Saul J.|last39=Njoroge|first39=Peter|last40=Dumbacher|first40=John Philip|last41=Ryder|first41=Oliver A.|last42=Fuchs|first42=Jérôme|last43=Bunce|first43=Michael|last44=Burt|first44=David W.|last45=Cracraft|first45=Joel|last46=Meng|first46=Guanliang|last47=Hackett|first47=Shannon J.|last48=Ryan|first48=Peter G.|last49=Jønsson|first49=Knud Andreas|last50=Jamieson|first50=Ian G.|last51=da Fonseca|first51=Rute R.|last52=Braun|first52=Edward L.|last53=Houde|first53=Peter|last54=Mirarab|first54=Siavash|last55=Suh|first55=Alexander|last56=Hansson|first56=Bengt|last57=Ponnikas|first57=Suvi|last58=Sigeman|first58=Hanna|last59=Stervander|first59=Martin|last60=Frandsen|first60=Paul B.|last61=van der Zwan|first61=Henriette|last62=van der Sluis|first62=Rencia|last63=Visser|first63=Carina|last64=Balakrishnan|first64=Christopher N.|last65=Clark|first65=Andrew G.|last66=Fitzpatrick|first66=John W.|last67=Bowman|first67=Reed|last68=Chen|first68=Nancy|last69=Cloutier|first69=Alison|last70=Sackton|first70=Timothy B.|last71=Edwards|first71=Scott V.|last72=Foote|first72=Dustin J.|last73=Shakya|first73=Subir B.|last74=Sheldon|first74=Frederick H.|last75=Vignal|first75=Alain|last76=Soares|first76=André E. 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Brandt|last123=Camenisch|first123=Glauco|last124=Keller|first124=Lukas F.|last125=DaCosta|first125=Jeffrey M.|last126=Hauber|first126=Mark E.|last127=Louder|first127=Matthew I. M.|last128=Witt|first128=Christopher C.|last129=McGuire|first129=Jimmy A.|last130=Mudge|first130=Joann|last131=Megna|first131=Libby C.|last132=Carling|first132=Matthew D.|last133=Wang|first133=Biao|last134=Taylor|first134=Scott A.|last135=Del-Rio|first135=Glaucia|last136=Aleixo|first136=Alexandre|last137=Vasconcelos|first137=Ana Tereza Ribeiro|last138=Mello|first138=Claudio V.|last139=Weir|first139=Jason T.|last140=Haussler|first140=David|last141=Li|first141=Qiye|last142=Yang|first142=Huanming|last143=Wang|first143=Jian|last144=Lei|first144=Fumin|last145=Rahbek|first145=Carsten|last146=Gilbert|first146=M. Thomas P.|last147=Graves|first147=Gary R.|last148=Jarvis|first148=Erich D.|last149=Paten|first149=Benedict|last150=Zhang|first150=Guojie|title=Dense sampling of bird diversity increases power of comparative genomics|journal=Nature|volume=587|issue=7833|year=2020|pages=252–257|issn=0028-0836|doi=10.1038/s41586-020-2873-9|pmid=33177665|pmc=7759463|bibcode=2020Natur.587..252F|doi-access=free}}</ref>
These include at least one species in about 90% of extant avian families (218 out of 236 families recognised by the ''Howard and Moore Checklist'').<ref name="FengStiller2020">{{cite journal|last1=Feng|first1=Shaohong|last2=Stiller|first2=Josefin|last3=Deng|first3=Yuan|last4=Armstrong|first4=Joel|last5=Fang|first5=Qi|last6=Reeve|first6=Andrew Hart|last7=Xie|first7=Duo|last8=Chen|first8=Guangji|last9=Guo|first9=Chunxue|last10=Faircloth|first10=Brant C.|last11=Petersen|first11=Bent|last12=Wang|first12=Zongji|last13=Zhou|first13=Qi|last14=Diekhans|first14=Mark|last15=Chen|first15=Wanjun|last16=Andreu-Sánchez|first16=Sergio|last17=Margaryan|first17=Ashot|last18=Howard|first18=Jason Travis|last19=Parent|first19=Carole|last20=Pacheco|first20=George|last21=Sinding|first21=Mikkel-Holger S.|last22=Puetz|first22=Lara|last23=Cavill|first23=Emily|last24=Ribeiro|first24=Ângela M.|last25=Eckhart|first25=Leopold|last26=Fjeldså|first26=Jon|last27=Hosner|first27=Peter A.|last28=Brumfield|first28=Robb T.|last29=Christidis|first29=Les|last30=Bertelsen|first30=Mads F.|last31=Sicheritz-Ponten|first31=Thomas|last32=Tietze|first32=Dieter Thomas|last33=Robertson|first33=Bruce C.|last34=Song|first34=Gang|last35=Borgia|first35=Gerald|last36=Claramunt|first36=Santiago|last37=Lovette|first37=Irby J.|last38=Cowen|first38=Saul J.|last39=Njoroge|first39=Peter|last40=Dumbacher|first40=John Philip|last41=Ryder|first41=Oliver A.|last42=Fuchs|first42=Jérôme|last43=Bunce|first43=Michael|last44=Burt|first44=David W.|last45=Cracraft|first45=Joel|last46=Meng|first46=Guanliang|last47=Hackett|first47=Shannon J.|last48=Ryan|first48=Peter G.|last49=Jønsson|first49=Knud Andreas|last50=Jamieson|first50=Ian G.|last51=da Fonseca|first51=Rute R.|last52=Braun|first52=Edward L.|last53=Houde|first53=Peter|last54=Mirarab|first54=Siavash|last55=Suh|first55=Alexander|last56=Hansson|first56=Bengt|last57=Ponnikas|first57=Suvi|last58=Sigeman|first58=Hanna|last59=Stervander|first59=Martin|last60=Frandsen|first60=Paul B.|last61=van der Zwan|first61=Henriette|last62=van der Sluis|first62=Rencia|last63=Visser|first63=Carina|last64=Balakrishnan|first64=Christopher N.|last65=Clark|first65=Andrew G.|last66=Fitzpatrick|first66=John W.|last67=Bowman|first67=Reed|last68=Chen|first68=Nancy|last69=Cloutier|first69=Alison|last70=Sackton|first70=Timothy B.|last71=Edwards|first71=Scott V.|last72=Foote|first72=Dustin J.|last73=Shakya|first73=Subir B.|last74=Sheldon|first74=Frederick H.|last75=Vignal|first75=Alain|last76=Soares|first76=André E. R.|last77=Shapiro|first77=Beth|last78=González-Solís|first78=Jacob|last79=Ferrer-Obiol|first79=Joan|last80=Rozas|first80=Julio|last81=Riutort|first81=Marta|last82=Tigano|first82=Anna|last83=Friesen|first83=Vicki|last84=Dalén|first84=Love|last85=Urrutia|first85=Araxi O.|last86=Székely|first86=Tamás|last87=Liu|first87=Yang|last88=Campana|first88=Michael G.|last89=Corvelo|first89=André|last90=Fleischer|first90=Robert C.|last91=Rutherford|first91=Kim M.|last92=Gemmell|first92=Neil J.|last93=Dussex|first93=Nicolas|last94=Mouritsen|first94=Henrik|last95=Thiele|first95=Nadine|last96=Delmore|first96=Kira|last97=Liedvogel|first97=Miriam|last98=Franke|first98=Andre|last99=Hoeppner|first99=Marc P.|display-authors=1|last100=Krone|first100=Oliver|last101=Fudickar|first101=Adam M.|last102=Milá|first102=Borja|last103=Ketterson|first103=Ellen D.|last104=Fidler|first104=Andrew Eric|last105=Friis|first105=Guillermo|last106=Parody-Merino|first106=Ángela M.|last107=Battley|first107=Phil F.|last108=Cox|first108=Murray P.|last109=Lima|first109=Nicholas Costa Barroso|last110=Prosdocimi|first110=Francisco|last111=Parchman|first111=Thomas Lee|last112=Schlinger|first112=Barney A.|last113=Loiselle|first113=Bette A.|last114=Blake|first114=John G.|last115=Lim|first115=Haw Chuan|last116=Day|first116=Lainy B.|last117=Fuxjager|first117=Matthew J.|last118=Baldwin|first118=Maude W.|last119=Braun|first119=Michael J.|last120=Wirthlin|first120=Morgan|last121=Dikow|first121=Rebecca B.|last122=Ryder|first122=T. Brandt|last123=Camenisch|first123=Glauco|last124=Keller|first124=Lukas F.|last125=DaCosta|first125=Jeffrey M.|last126=Hauber|first126=Mark E.|last127=Louder|first127=Matthew I. M.|last128=Witt|first128=Christopher C.|last129=McGuire|first129=Jimmy A.|last130=Mudge|first130=Joann|last131=Megna|first131=Libby C.|last132=Carling|first132=Matthew D.|last133=Wang|first133=Biao|last134=Taylor|first134=Scott A.|last135=Del-Rio|first135=Glaucia|last136=Aleixo|first136=Alexandre|last137=Vasconcelos|first137=Ana Tereza Ribeiro|last138=Mello|first138=Claudio V.|last139=Weir|first139=Jason T.|last140=Haussler|first140=David|last141=Li|first141=Qiye|last142=Yang|first142=Huanming|last143=Wang|first143=Jian|last144=Lei|first144=Fumin|last145=Rahbek|first145=Carsten|last146=Gilbert|first146=M. Thomas P.|last147=Graves|first147=Gary R.|last148=Jarvis|first148=Erich D.|last149=Paten|first149=Benedict|last150=Zhang|first150=Guojie|title=Dense sampling of bird diversity increases power of comparative genomics|journal=Nature|volume=587|issue=7833|year=2020|pages=252–257|issn=0028-0836|doi=10.1038/s41586-020-2873-9|pmid=33177665|pmc=7759463|bibcode=2020Natur.587..252F|doi-access=free}}</ref>


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===Reproductive system===
===Reproductive system===
Males within [[Palaeognathae]] (with the exception of the [[Kiwi (bird)|kiwi]]s), the [[Anseriformes]] (with the exception of [[screamer]]s), and in rudimentary forms in [[Galliformes]] (but fully developed in [[Cracidae]]) possess a [[bird penis|penis]], which is never present in [[Neoaves]].<ref>{{cite web|last=Yong |first=Ed |url=http://phenomena.nationalgeographic.com/2013/06/06/how-chickens-lost-their-penises-ducks-kept-theirs/ |title=Phenomena: Not Exactly Rocket Science How Chickens Lost Their Penises (And Ducks Kept Theirs) |date=6 June 2013 |publisher=Phenomena.nationalgeographic.com |access-date=3 October 2013}}</ref><ref>{{cite web |url=http://bcs.whfreeman.com/gill/bcs-pages/body-right_10.asp?s=10000&n=00010&i=99010.06&v=chapter&o=%7C13000%7C00010%7C&ns=undefined |title=Ornithology, 3rd Edition – Waterfowl: Order Anseriformes |access-date=3 October 2013 |url-status=dead |archive-url=https://web.archive.org/web/20150622030534/http://bcs.whfreeman.com/gill/bcs-pages/body-right_10.asp?s=10000&n=00010&i=99010.06&v=chapter&o=%7C13000%7C00010%7C&ns=undefined |archive-date=22 June 2015}}</ref> The length is thought to be related to [[sperm competition]].<ref>{{cite journal|journal=The Auk |volume=117 |issue=3 |pages=820–825 |year=2000 |title=The 20-cm Spiny Penis of the Argentine Lake Duck (Oxyura vittata) |author=McCracken, KG |url=http://sora.unm.edu/sites/default/files/journals/auk/v117n03/p00820-p00825.pdf |archive-url=https://web.archive.org/web/20160304210246/http://sora.unm.edu/sites/default/files/journals/auk/v117n03/p00820-p00825.pdf |archive-date=4 March 2016 |doi=10.1642/0004-8038(2000)117[0820:TCSPOT]2.0.CO;2 |url-status=dead}}</ref> When not copulating, it is hidden within the [[proctodeum]] compartment within the cloaca, just inside the vent. Female birds have [[Female sperm storage|sperm storage]] tubules<ref>{{Cite journal|last1=Sasanami|first1=Tomohiro|last2=Matsuzaki|first2=Mei|last3=Mizushima|first3=Shusei|last4=Hiyama|first4=Gen|date=2013|title=Sperm Storage in the Female Reproductive Tract in Birds|journal=Journal of Reproduction and Development|language=en|volume=59|issue=4|pages=334–338|doi=10.1262/jrd.2013-038|issn=0916-8818|pmc=3944358|pmid=23965601}}</ref> that allow sperm to remain viable long after copulation, a hundred days in some species.<ref>{{cite journal|last1=Birkhead|first1=T.R.|last2=Møller|first2=P.|year=1993|title=Sexual selection and the temporal separation of reproductive events: sperm storage data from reptiles, birds and mammals|journal=Biological Journal of the Linnean Society|volume=50|issue=4|pages=295–311|doi=10.1111/j.1095-8312.1993.tb00933.x}}</ref> Sperm from multiple males may [[Sperm competition|compete]] through this mechanism. Most female birds have a single [[ovary]] and a single [[oviduct]], both on the left side,<ref name="karger">{{Cite journal|last1=Guioli|first1=Silvana|last2=Nandi|first2=Sunil|last3=Zhao|first3=Debiao|last4=Burgess-Shannon|first4=Jessica|last5=Lovell-Badge|first5=Robin|last6=Clinton|first6=Michael|date=2014|title=Gonadal Asymmetry and Sex Determination in Birds|url=https://www.karger.com/Article/FullText/358406|journal=Sexual Development|language=en|volume=8|issue=5|pages=227–242|doi=10.1159/000358406|pmid=24577119|s2cid=3035039|issn=1661-5433|doi-access=free}}</ref> but there are exceptions: species in at least 16 different orders of birds have two ovaries. Even these species, however, tend to have a single oviduct.<ref name="karger" /> It has been speculated that this might be an adaptation to flight, but males have two testes, and it is also observed that the gonads in both sexes decrease dramatically in size outside the breeding season.<ref>{{cite journal |last1=Dawson |first1=Alistair |title=Annual gonadal cycles in birds: Modeling the effects of photoperiod on seasonal changes in GnRH-1 secretion |journal=Frontiers in Neuroendocrinology |date=April 2015 |volume=37 |pages=52–64 |doi=10.1016/j.yfrne.2014.08.004|pmid=25194876 |s2cid=13704885 |doi-access=free }}</ref><ref>{{cite journal |last1=FARNER |first1=DONALD S. |last2=FOLLETT |first2=BRIAN K. |last3=KING |first3=JAMESR. |last4=MORTON |first4=MARTIN L. |title=A Quantitative Examination of Ovarian Growth in the White-Crowned Sparrow |journal=The Biological Bulletin |date=February 1966 |volume=130 |issue=1 |pages=67–75 |doi=10.2307/1539953|jstor=1539953 |pmid=5948479 |url=https://www.biodiversitylibrary.org/part/9389 }}</ref> Also terrestrial birds generally have a single ovary, as does the [[platypus]], an egg-laying mammal. A more likely explanation is that the egg develops a shell while passing through the oviduct over a period of about a day, so that if two eggs were to develop at the same time, there would be a risk to survival.<ref name="karger" /> While rare, mostly abortive, [[parthenogenesis]] is not unknown in birds and eggs can be [[Ploidy#Diploid|diploid]], [[automixis|automictic]] and results in male offspring.<ref>{{Cite journal|last1=Ramachandran|first1=R|last2=McDaniel|first2=C D|date=2018|title=Parthenogenesis in birds: a review|url=https://rep.bioscientifica.com/view/journals/rep/155/6/REP-17-0728.xml|journal=Reproduction|volume=155|issue=6|pages=R245–R257|doi=10.1530/REP-17-0728|pmid=29559496|s2cid=4017618|issn=1470-1626}}</ref>
Males within [[Palaeognathae]] (with the exception of the [[Kiwi (bird)|kiwi]]s), the [[Anseriformes]] (with the exception of [[screamer]]s), and in rudimentary forms in [[Galliformes]] (but fully developed in [[Cracidae]]) possess a [[bird penis|penis]], which is never present in [[Neoaves]].<ref>{{cite web|last=Yong |first=Ed |url=http://phenomena.nationalgeographic.com/2013/06/06/how-chickens-lost-their-penises-ducks-kept-theirs/ |title=Phenomena: Not Exactly Rocket Science How Chickens Lost Their Penises (And Ducks Kept Theirs) |date=6 June 2013 |publisher=Phenomena.nationalgeographic.com |access-date=3 October 2013}}</ref><ref>{{cite web |url=http://bcs.whfreeman.com/gill/bcs-pages/body-right_10.asp?s=10000&n=00010&i=99010.06&v=chapter&o=%7C13000%7C00010%7C&ns=undefined |title=Ornithology, 3rd Edition – Waterfowl: Order Anseriformes |access-date=3 October 2013 |url-status=dead |archive-url=https://web.archive.org/web/20150622030534/http://bcs.whfreeman.com/gill/bcs-pages/body-right_10.asp?s=10000&n=00010&i=99010.06&v=chapter&o=%7C13000%7C00010%7C&ns=undefined |archive-date=22 June 2015}}</ref> The length is thought to be related to [[sperm competition]].<ref>{{cite journal|journal=The Auk |volume=117 |issue=3 |pages=820–825 |year=2000 |title=The 20-cm Spiny Penis of the Argentine Lake Duck (Oxyura vittata) |author=McCracken, KG |url=http://sora.unm.edu/sites/default/files/journals/auk/v117n03/p00820-p00825.pdf |archive-url=https://web.archive.org/web/20160304210246/http://sora.unm.edu/sites/default/files/journals/auk/v117n03/p00820-p00825.pdf |archive-date=4 March 2016 |doi=10.1642/0004-8038(2000)117[0820:TCSPOT]2.0.CO;2 |s2cid=5717257 |url-status=dead}}</ref> When not copulating, it is hidden within the [[proctodeum]] compartment within the cloaca, just inside the vent. Female birds have [[Female sperm storage|sperm storage]] tubules<ref>{{Cite journal|last1=Sasanami|first1=Tomohiro|last2=Matsuzaki|first2=Mei|last3=Mizushima|first3=Shusei|last4=Hiyama|first4=Gen|date=2013|title=Sperm Storage in the Female Reproductive Tract in Birds|journal=Journal of Reproduction and Development|language=en|volume=59|issue=4|pages=334–338|doi=10.1262/jrd.2013-038|issn=0916-8818|pmc=3944358|pmid=23965601}}</ref> that allow sperm to remain viable long after copulation, a hundred days in some species.<ref>{{cite journal|last1=Birkhead|first1=T.R.|last2=Møller|first2=P.|year=1993|title=Sexual selection and the temporal separation of reproductive events: sperm storage data from reptiles, birds and mammals|journal=Biological Journal of the Linnean Society|volume=50|issue=4|pages=295–311|doi=10.1111/j.1095-8312.1993.tb00933.x}}</ref> Sperm from multiple males may [[Sperm competition|compete]] through this mechanism. Most female birds have a single [[ovary]] and a single [[oviduct]], both on the left side,<ref name="karger">{{Cite journal|last1=Guioli|first1=Silvana|last2=Nandi|first2=Sunil|last3=Zhao|first3=Debiao|last4=Burgess-Shannon|first4=Jessica|last5=Lovell-Badge|first5=Robin|last6=Clinton|first6=Michael|date=2014|title=Gonadal Asymmetry and Sex Determination in Birds|url=https://www.karger.com/Article/FullText/358406|journal=Sexual Development|language=en|volume=8|issue=5|pages=227–242|doi=10.1159/000358406|pmid=24577119|s2cid=3035039|issn=1661-5433|doi-access=free}}</ref> but there are exceptions: species in at least 16 different orders of birds have two ovaries. Even these species, however, tend to have a single oviduct.<ref name="karger" /> It has been speculated that this might be an adaptation to flight, but males have two testes, and it is also observed that the gonads in both sexes decrease dramatically in size outside the breeding season.<ref>{{cite journal |last1=Dawson |first1=Alistair |title=Annual gonadal cycles in birds: Modeling the effects of photoperiod on seasonal changes in GnRH-1 secretion |journal=Frontiers in Neuroendocrinology |date=April 2015 |volume=37 |pages=52–64 |doi=10.1016/j.yfrne.2014.08.004|pmid=25194876 |s2cid=13704885 |doi-access=free }}</ref><ref>{{cite journal |last1=FARNER |first1=DONALD S. |last2=FOLLETT |first2=BRIAN K. |last3=KING |first3=JAMESR. |last4=MORTON |first4=MARTIN L. |title=A Quantitative Examination of Ovarian Growth in the White-Crowned Sparrow |journal=The Biological Bulletin |date=February 1966 |volume=130 |issue=1 |pages=67–75 |doi=10.2307/1539953|jstor=1539953 |pmid=5948479 |url=https://www.biodiversitylibrary.org/part/9389 }}</ref> Also terrestrial birds generally have a single ovary, as does the [[platypus]], an egg-laying mammal. A more likely explanation is that the egg develops a shell while passing through the oviduct over a period of about a day, so that if two eggs were to develop at the same time, there would be a risk to survival.<ref name="karger" /> While rare, mostly abortive, [[parthenogenesis]] is not unknown in birds and eggs can be [[Ploidy#Diploid|diploid]], [[automixis|automictic]] and results in male offspring.<ref>{{Cite journal|last1=Ramachandran|first1=R|last2=McDaniel|first2=C D|date=2018|title=Parthenogenesis in birds: a review|url=https://rep.bioscientifica.com/view/journals/rep/155/6/REP-17-0728.xml|journal=Reproduction|volume=155|issue=6|pages=R245–R257|doi=10.1530/REP-17-0728|pmid=29559496|s2cid=4017618|issn=1470-1626}}</ref>


Birds are solely [[Gonochorism|gonochoric]].<ref>{{Cite book|last1=Kobayashi|first1=Kazuya|url=https://books.google.com/books?id=g4teDwAAQBAJ&q=mammal+gonochorism&pg=PA290|title=Reproductive and Developmental Strategies: The Continuity of Life|last2=Kitano|first2=Takeshi|last3=Iwao|first3=Yasuhiro|last4=Kondo|first4=Mariko|date=2018-06-01|publisher=Springer|isbn=978-4-431-56609-0|pages=290|language=en}}</ref> Meaning they have two sexes: either [[female]] or [[male]]. The sex of birds is determined by the [[ZW sex-determination system|Z and W sex chromosomes]], rather than by the [[XY sex-determination system|X and Y chromosomes]] present in [[mammal]]s. Male birds have two Z chromosomes (ZZ), and female birds have a W chromosome and a Z chromosome (WZ).<ref name = "Gill"/>
Birds are solely [[Gonochorism|gonochoric]].<ref>{{Cite book|last1=Kobayashi|first1=Kazuya|url=https://books.google.com/books?id=g4teDwAAQBAJ&q=mammal+gonochorism&pg=PA290|title=Reproductive and Developmental Strategies: The Continuity of Life|last2=Kitano|first2=Takeshi|last3=Iwao|first3=Yasuhiro|last4=Kondo|first4=Mariko|date=2018-06-01|publisher=Springer|isbn=978-4-431-56609-0|pages=290|language=en}}</ref> Meaning they have two sexes: either [[female]] or [[male]]. The sex of birds is determined by the [[ZW sex-determination system|Z and W sex chromosomes]], rather than by the [[XY sex-determination system|X and Y chromosomes]] present in [[mammal]]s. Male birds have two Z chromosomes (ZZ), and female birds have a W chromosome and a Z chromosome (WZ).<ref name = "Gill"/>
Line 492: Line 492:
Most birds can [[Flying and gliding animals|fly]], which distinguishes them from almost all other vertebrate classes. Flight is the primary means of locomotion for most bird species and is used for searching for food and for escaping from predators. Birds have various adaptations for flight, including a lightweight skeleton, two large flight muscles, the pectoralis (which accounts for 15% of the total mass of the bird) and the supracoracoideus, as well as a modified forelimb ([[wing]]) that serves as an [[airfoil|aerofoil]].<ref name = "Gill"/>
Most birds can [[Flying and gliding animals|fly]], which distinguishes them from almost all other vertebrate classes. Flight is the primary means of locomotion for most bird species and is used for searching for food and for escaping from predators. Birds have various adaptations for flight, including a lightweight skeleton, two large flight muscles, the pectoralis (which accounts for 15% of the total mass of the bird) and the supracoracoideus, as well as a modified forelimb ([[wing]]) that serves as an [[airfoil|aerofoil]].<ref name = "Gill"/>


Wing shape and size generally determine a bird's flight style and performance; many birds combine powered, flapping flight with less energy-intensive soaring flight. About 60 extant bird species are [[Flightless bird|flightless]], as were many extinct birds.<ref>{{Cite book|last=Roots |first=Clive |year=2006 |title=Flightless Birds |location=Westport |publisher=Greenwood Press |isbn=978-0-313-33545-7}}</ref> Flightlessness often arises in birds on isolated islands, most likely due to limited resources and the absence of [[mammal]]ian land predators.<ref>{{Cite journal|last=McNab |first=Brian K. |date=October 1994 |title=Energy Conservation and the Evolution of Flightlessness in Birds |journal=The American Naturalist |volume=144 |issue=4 |pages=628–642 |doi=10.1086/285697 |jstor=2462941|s2cid=86511951 }}</ref> Flightlessnes is almost exclusively correlated with [[Island gigantism|gigantism]] due to an island's inheren condition of isolation.<ref>{{Cite web|url=https://www.sciencedirect.com/topics/earth-and-planetary-sciences/flightlessness|title = Flightlessness - an overview &#124; ScienceDirect Topics}}</ref> Although flightless, penguins use similar musculature and movements to "fly" through the water, as do some flight-capable birds such as [[auk]]s, [[shearwater]]s and [[dipper]]s.<ref>{{Cite journal|last1=Kovacs |first1=Christopher E. |year=2000 |title=Anatomy and histochemistry of flight muscles in a wing-propelled diving bird, the Atlantic Puffin, ''Fratercula arctica'' |journal=Journal of Morphology |volume=244 |issue=2 |pages=109–125|doi=10.1002/(SICI)1097-4687(200005)244:2<109::AID-JMOR2>3.0.CO;2-0 |pmid=10761049 |last2=Meyers |first2=RA}}</ref>
Wing shape and size generally determine a bird's flight style and performance; many birds combine powered, flapping flight with less energy-intensive soaring flight. About 60 extant bird species are [[Flightless bird|flightless]], as were many extinct birds.<ref>{{Cite book|last=Roots |first=Clive |year=2006 |title=Flightless Birds |location=Westport |publisher=Greenwood Press |isbn=978-0-313-33545-7}}</ref> Flightlessness often arises in birds on isolated islands, most likely due to limited resources and the absence of [[mammal]]ian land predators.<ref>{{Cite journal|last=McNab |first=Brian K. |date=October 1994 |title=Energy Conservation and the Evolution of Flightlessness in Birds |journal=The American Naturalist |volume=144 |issue=4 |pages=628–642 |doi=10.1086/285697 |jstor=2462941|s2cid=86511951 }}</ref> Flightlessnes is almost exclusively correlated with [[Island gigantism|gigantism]] due to an island's inheren condition of isolation.<ref>{{Cite web|url=https://www.sciencedirect.com/topics/earth-and-planetary-sciences/flightlessness|title = Flightlessness - an overview &#124; ScienceDirect Topics}}</ref> Although flightless, penguins use similar musculature and movements to "fly" through the water, as do some flight-capable birds such as [[auk]]s, [[shearwater]]s and [[dipper]]s.<ref>{{Cite journal|last1=Kovacs |first1=Christopher E. |year=2000 |title=Anatomy and histochemistry of flight muscles in a wing-propelled diving bird, the Atlantic Puffin, ''Fratercula arctica'' |journal=Journal of Morphology |volume=244 |issue=2 |pages=109–125|doi=10.1002/(SICI)1097-4687(200005)244:2<109::AID-JMOR2>3.0.CO;2-0 |pmid=10761049 |last2=Meyers |first2=RA|s2cid=14041453 }}</ref>
{{Clear}}
{{Clear}}


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===Feather care===
===Feather care===
{{Main|Preening}}
{{Main|Preening}}
Feathers, being critical to the survival of a bird, require maintenance. Apart from physical wear and tear, feathers face the onslaught of fungi, [[ectoparasitic]] feather mites and [[Bird louse|bird lice]].<ref>{{cite journal|title=The alterations of plumage of parasitic origin|first1=Principato|last1=Mario|first2=Lisi|last2=Federica|first3=Moretta|last3=Iolanda|first4=Samra|last4=Nada|first5=Puccetti|last5=Francesco|journal=Italian Journal of Animal Science|volume=4|issue=3|pages=296–299|year=2005|doi=10.4081/ijas.2005.296|s2cid=84770232|doi-access=free}}</ref> The physical condition of feathers are maintained by {{Birdgloss|preening}} often with the application of secretions from the {{Birdgloss|preen gland}}. Birds also bathe in water or dust themselves. While some birds dip into shallow water, more aerial species may make aerial dips into water and arboreal species often make use of dew or rain that collect on leaves. Birds of arid regions make use of loose soil to dust-bathe. A behaviour termed as [[Anting (bird activity)|anting]] in which the bird encourages ants to run through their plumage is also thought to help them reduce the ectoparasite load in feathers. Many species will spread out their wings and expose them to direct sunlight and this too is thought to help in reducing fungal and ectoparasitic activity that may lead to feather damage.<ref>{{cite journal|journal=The Auk |volume=121|issue=4|pages=1262–1268| year=2004| doi=10.1642/0004-8038(2004)121[1262:BAFAOA]2.0.CO;2| title=Bactericidal and fungicidal activity of ant chemicals on feather parasites: an evaluation of anting behavior as a method of self-medication in songbirds| first1=Hannah C. |last1=Revis|first2=Deborah A. |last2=Waller}}</ref><ref>{{cite journal|journal=The Open Ornithology Journal|year=2010|volume=3|pages=41–71|doi=10.2174/1874453201003010041|title=How Birds Combat Ectoparasites|first1=Dale H.|last1=Clayton|first2=Jennifer A.H.|last2=Koop|first3=Christopher W.|last3=Harbison|first4=Brett R.|last4=Moyer|first5=Sarah E.|last5=Bush|doi-access=free}}</ref>
Feathers, being critical to the survival of a bird, require maintenance. Apart from physical wear and tear, feathers face the onslaught of fungi, [[ectoparasitic]] feather mites and [[Bird louse|bird lice]].<ref>{{cite journal|title=The alterations of plumage of parasitic origin|first1=Principato|last1=Mario|first2=Lisi|last2=Federica|first3=Moretta|last3=Iolanda|first4=Samra|last4=Nada|first5=Puccetti|last5=Francesco|journal=Italian Journal of Animal Science|volume=4|issue=3|pages=296–299|year=2005|doi=10.4081/ijas.2005.296|s2cid=84770232|doi-access=free}}</ref> The physical condition of feathers are maintained by {{Birdgloss|preening}} often with the application of secretions from the {{Birdgloss|preen gland}}. Birds also bathe in water or dust themselves. While some birds dip into shallow water, more aerial species may make aerial dips into water and arboreal species often make use of dew or rain that collect on leaves. Birds of arid regions make use of loose soil to dust-bathe. A behaviour termed as [[Anting (bird activity)|anting]] in which the bird encourages ants to run through their plumage is also thought to help them reduce the ectoparasite load in feathers. Many species will spread out their wings and expose them to direct sunlight and this too is thought to help in reducing fungal and ectoparasitic activity that may lead to feather damage.<ref>{{cite journal|journal=The Auk |volume=121|issue=4|pages=1262–1268| year=2004| doi=10.1642/0004-8038(2004)121[1262:BAFAOA]2.0.CO;2| title=Bactericidal and fungicidal activity of ant chemicals on feather parasites: an evaluation of anting behavior as a method of self-medication in songbirds| first1=Hannah C. |last1=Revis|first2=Deborah A. |last2=Waller|s2cid=85677766 }}</ref><ref>{{cite journal|journal=The Open Ornithology Journal|year=2010|volume=3|pages=41–71|doi=10.2174/1874453201003010041|title=How Birds Combat Ectoparasites|first1=Dale H.|last1=Clayton|first2=Jennifer A.H.|last2=Koop|first3=Christopher W.|last3=Harbison|first4=Brett R.|last4=Moyer|first5=Sarah E.|last5=Bush|doi-access=free}}</ref>


===Migration===
===Migration===
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[[File:Bar-tailed Godwit migration.jpg|alt= A map of the Pacific Ocean with several coloured lines representing bird routes running from New Zealand to Korea|thumb|left|The routes of satellite-tagged [[bar-tailed godwit]]s migrating north from [[New Zealand]]. This species has the longest known non-stop migration of any species, up to {{convert|10200|km|mi|-2|abbr=on}}.]]
[[File:Bar-tailed Godwit migration.jpg|alt= A map of the Pacific Ocean with several coloured lines representing bird routes running from New Zealand to Korea|thumb|left|The routes of satellite-tagged [[bar-tailed godwit]]s migrating north from [[New Zealand]]. This species has the longest known non-stop migration of any species, up to {{convert|10200|km|mi|-2|abbr=on}}.]]
Some bird species undertake shorter migrations, travelling only as far as is required to avoid bad weather or obtain food. [[wikt:irruptive|Irruptive]] species such as the boreal [[finch]]es are one such group and can commonly be found at a location in one year and absent the next. This type of migration is normally associated with food availability.<ref>{{Cite journal|last=Wilson |first=W. Herbert, Jr. |year=1999 |title=Bird feeding and irruptions of northern finches:are migrations short stopped? |journal=North America Bird Bander |volume=24 |issue=4 |pages=113–121 |url=http://sora.unm.edu/sites/default/files/journals/nabb/v024n04/p0113-p0121.pdf |archive-url=https://web.archive.org/web/20140729162642/https://sora.unm.edu/sites/default/files/journals/nabb/v024n04/p0113-p0121.pdf |archive-date=29 July 2014 |url-status=dead}}</ref> Species may also travel shorter distances over part of their range, with individuals from higher latitudes travelling into the existing range of conspecifics; others undertake partial migrations, where only a fraction of the population, usually females and subdominant males, migrates.<ref>{{Cite journal|last1=Nilsson |first1=Anna L.K. |year=2006 |title=Do partial and regular migrants differ in their responses to weather? |journal=The Auk |volume=123 |issue=2 |pages=537–547 |doi=10.1642/0004-8038(2006)123[537:DPARMD]2.0.CO;2 |last2=Alerstam |first2=Thomas |last3=Nilsson |first3=Jan-Åke}}</ref> Partial migration can form a large percentage of the migration behaviour of birds in some regions; in Australia, surveys found that 44% of non-passerine birds and 32% of passerines were partially migratory.<ref>{{Cite journal|last=Chan |first=Ken |year=2001 |title=Partial migration in Australian landbirds: a review |journal=[[Emu (journal)|Emu]] |volume=101 |issue=4 |pages=281–292 |doi=10.1071/MU00034|s2cid=82259620 }}</ref>
Some bird species undertake shorter migrations, travelling only as far as is required to avoid bad weather or obtain food. [[wikt:irruptive|Irruptive]] species such as the boreal [[finch]]es are one such group and can commonly be found at a location in one year and absent the next. This type of migration is normally associated with food availability.<ref>{{Cite journal|last=Wilson |first=W. Herbert, Jr. |year=1999 |title=Bird feeding and irruptions of northern finches:are migrations short stopped? |journal=North America Bird Bander |volume=24 |issue=4 |pages=113–121 |url=http://sora.unm.edu/sites/default/files/journals/nabb/v024n04/p0113-p0121.pdf |archive-url=https://web.archive.org/web/20140729162642/https://sora.unm.edu/sites/default/files/journals/nabb/v024n04/p0113-p0121.pdf |archive-date=29 July 2014 |url-status=dead}}</ref> Species may also travel shorter distances over part of their range, with individuals from higher latitudes travelling into the existing range of conspecifics; others undertake partial migrations, where only a fraction of the population, usually females and subdominant males, migrates.<ref>{{Cite journal|last1=Nilsson |first1=Anna L.K. |year=2006 |title=Do partial and regular migrants differ in their responses to weather? |journal=The Auk |volume=123 |issue=2 |pages=537–547 |doi=10.1642/0004-8038(2006)123[537:DPARMD]2.0.CO;2 |last2=Alerstam |first2=Thomas |last3=Nilsson |first3=Jan-Åke|s2cid=84665086 }}</ref> Partial migration can form a large percentage of the migration behaviour of birds in some regions; in Australia, surveys found that 44% of non-passerine birds and 32% of passerines were partially migratory.<ref>{{Cite journal|last=Chan |first=Ken |year=2001 |title=Partial migration in Australian landbirds: a review |journal=[[Emu (journal)|Emu]] |volume=101 |issue=4 |pages=281–292 |doi=10.1071/MU00034|s2cid=82259620 }}</ref>


[[Altitudinal migration]] is a form of short-distance migration in which birds spend the breeding season at higher altitudes and move to lower ones during suboptimal conditions. It is most often triggered by temperature changes and usually occurs when [[territory (animal)|the normal territories]] also become inhospitable due to lack of food.<ref>{{Cite journal|last=Rabenold |first=Kerry N. |year=1985 |title=Variation in Altitudinal Migration, Winter Segregation, and Site Tenacity in two subspecies of Dark-eyed Juncos in the southern Appalachians |journal=The Auk |volume=102 |issue=4 |pages=805–819 |url=http://sora.unm.edu/sites/default/files/journals/auk/v102n04/p0805-p0819.pdf}}</ref> Some species may also be nomadic, holding no fixed territory and moving according to weather and food availability. [[True parrots|Parrots]] as a [[family (biology)|family]] are overwhelmingly neither migratory nor sedentary but considered to either be dispersive, irruptive, nomadic or undertake small and irregular migrations.<ref>{{Cite book|last=Collar |first=Nigel J. |year=1997 |chapter=Family Psittacidae (Parrots) |title=Handbook of the Birds of the World |volume=4: Sandgrouse to Cuckoos |editor=Josep del Hoyo |editor2=Andrew Elliott |editor3=Jordi Sargatal |location=Barcelona |publisher=Lynx Edicions |isbn=84-87334-22-9|title-link=Handbook of the Birds of the World }}</ref>
[[Altitudinal migration]] is a form of short-distance migration in which birds spend the breeding season at higher altitudes and move to lower ones during suboptimal conditions. It is most often triggered by temperature changes and usually occurs when [[territory (animal)|the normal territories]] also become inhospitable due to lack of food.<ref>{{Cite journal|last=Rabenold |first=Kerry N. |year=1985 |title=Variation in Altitudinal Migration, Winter Segregation, and Site Tenacity in two subspecies of Dark-eyed Juncos in the southern Appalachians |journal=The Auk |volume=102 |issue=4 |pages=805–819 |url=http://sora.unm.edu/sites/default/files/journals/auk/v102n04/p0805-p0819.pdf}}</ref> Some species may also be nomadic, holding no fixed territory and moving according to weather and food availability. [[True parrots|Parrots]] as a [[family (biology)|family]] are overwhelmingly neither migratory nor sedentary but considered to either be dispersive, irruptive, nomadic or undertake small and irregular migrations.<ref>{{Cite book|last=Collar |first=Nigel J. |year=1997 |chapter=Family Psittacidae (Parrots) |title=Handbook of the Birds of the World |volume=4: Sandgrouse to Cuckoos |editor=Josep del Hoyo |editor2=Andrew Elliott |editor3=Jordi Sargatal |location=Barcelona |publisher=Lynx Edicions |isbn=84-87334-22-9|title-link=Handbook of the Birds of the World }}</ref>
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[[Bird vocalization|Bird calls and songs]], which are produced in the [[Syrinx (biology)|syrinx]], are the major means by which birds communicate with [[sound]]. This communication can be very complex; some species can operate the two sides of the syrinx independently, allowing the simultaneous production of two different songs.<ref name = "Suthers"/>
[[Bird vocalization|Bird calls and songs]], which are produced in the [[Syrinx (biology)|syrinx]], are the major means by which birds communicate with [[sound]]. This communication can be very complex; some species can operate the two sides of the syrinx independently, allowing the simultaneous production of two different songs.<ref name = "Suthers"/>
Calls are used for a variety of purposes, including mate attraction,<ref name = "Gill"/> evaluation of potential mates,<ref>{{Cite journal |doi=10.1080/08927014.1994.9522988 |last1=Genevois |first1=F. |year=1994 |last2=Bretagnolle |first2=V. |title=Male Blue Petrels reveal their body mass when calling |journal=Ethology Ecology and Evolution |volume=6 |issue=3 |pages=377–383 |url=http://ejour-fup.unifi.it/index.php/eee/article/view/667/613 |url-status=dead |archive-url=https://web.archive.org/web/20071224040649/http://ejour-fup.unifi.it/index.php/eee/article/view/667/613 |archive-date=24 December 2007}}</ref> bond formation, the claiming and maintenance of territories,<ref name = "Gill"/> the identification of other individuals (such as when parents look for chicks in colonies or when mates reunite at the start of breeding season),<ref>{{Cite journal|last1=Jouventin |first1=Pierre |date=June 1999 |title=Finding a parent in a king penguin colony: the acoustic system of individual recognition |journal=Animal Behaviour |volume=57 |issue=6 |pages=1175–1183 |doi=10.1006/anbe.1999.1086 |pmid=10373249 |last2=Aubin |first2=T |last3=Lengagne |first3=T|s2cid=45578269 }}</ref> and the warning of other birds of potential predators, sometimes with specific information about the nature of the threat.<ref>{{Cite journal|last1=Templeton |first1=Christopher N. |year=2005 |title=Allometry of Alarm Calls: Black-Capped Chickadees Encode Information About Predator Size |journal=Science |volume=308 |issue=5730 |pages=1934–1937 |doi=10.1126/science.1108841 |pmid=15976305 |last2=Greene |first2=E |last3=Davis |first3=K|bibcode=2005Sci...308.1934T |s2cid=42276496 }}</ref> Some birds also use mechanical sounds for auditory communication. The ''[[Coenocorypha]]'' [[snipe]]s of [[New Zealand]] drive air through their feathers,<ref name = "Miskelly">{{Cite journal|last=Miskelly |first=C.M. |date=July 1987 |title=The identity of the hakawai |journal=Notornis |volume=34 |issue=2 |pages=95–116}}</ref> [[woodpecker]]s drum for long-distance communication,<ref name="DodenhoffStark2001">{{cite journal|last1=Dodenhoff|first1=Danielle J.|last2=Stark|first2=Robert D.|last3=Johnson|first3=Eric V.|title=Do woodpecker drums encode information for species recognition?|journal=The Condor|volume=103|issue=1|year=2001|page=143|issn=0010-5422|doi=10.1650/0010-5422(2001)103[0143:DWDEIF]2.0.CO;2}}</ref> and [[palm cockatoo]]s use tools to drum.<ref>{{Cite journal|last1=Murphy |first1=Stephen |year=2003 |title=The breeding biology of palm cockatoos (''Probosciger aterrimus''): a case of a slow life history |journal=[[Journal of Zoology]] |volume=261 |issue=4 |pages=327–339 |doi=10.1017/S0952836903004175 |last2=Legge |first2=Sarah |last3=Heinsohn |first3=Robert}}</ref>
Calls are used for a variety of purposes, including mate attraction,<ref name = "Gill"/> evaluation of potential mates,<ref>{{Cite journal |doi=10.1080/08927014.1994.9522988 |last1=Genevois |first1=F. |year=1994 |last2=Bretagnolle |first2=V. |title=Male Blue Petrels reveal their body mass when calling |journal=Ethology Ecology and Evolution |volume=6 |issue=3 |pages=377–383 |url=http://ejour-fup.unifi.it/index.php/eee/article/view/667/613 |url-status=dead |archive-url=https://web.archive.org/web/20071224040649/http://ejour-fup.unifi.it/index.php/eee/article/view/667/613 |archive-date=24 December 2007}}</ref> bond formation, the claiming and maintenance of territories,<ref name = "Gill"/> the identification of other individuals (such as when parents look for chicks in colonies or when mates reunite at the start of breeding season),<ref>{{Cite journal|last1=Jouventin |first1=Pierre |date=June 1999 |title=Finding a parent in a king penguin colony: the acoustic system of individual recognition |journal=Animal Behaviour |volume=57 |issue=6 |pages=1175–1183 |doi=10.1006/anbe.1999.1086 |pmid=10373249 |last2=Aubin |first2=T |last3=Lengagne |first3=T|s2cid=45578269 }}</ref> and the warning of other birds of potential predators, sometimes with specific information about the nature of the threat.<ref>{{Cite journal|last1=Templeton |first1=Christopher N. |year=2005 |title=Allometry of Alarm Calls: Black-Capped Chickadees Encode Information About Predator Size |journal=Science |volume=308 |issue=5730 |pages=1934–1937 |doi=10.1126/science.1108841 |pmid=15976305 |last2=Greene |first2=E |last3=Davis |first3=K|bibcode=2005Sci...308.1934T |s2cid=42276496 }}</ref> Some birds also use mechanical sounds for auditory communication. The ''[[Coenocorypha]]'' [[snipe]]s of [[New Zealand]] drive air through their feathers,<ref name = "Miskelly">{{Cite journal|last=Miskelly |first=C.M. |date=July 1987 |title=The identity of the hakawai |journal=Notornis |volume=34 |issue=2 |pages=95–116}}</ref> [[woodpecker]]s drum for long-distance communication,<ref name="DodenhoffStark2001">{{cite journal|last1=Dodenhoff|first1=Danielle J.|last2=Stark|first2=Robert D.|last3=Johnson|first3=Eric V.|title=Do woodpecker drums encode information for species recognition?|journal=The Condor|volume=103|issue=1|year=2001|page=143|issn=0010-5422|doi=10.1650/0010-5422(2001)103[0143:DWDEIF]2.0.CO;2|s2cid=31878910 }}</ref> and [[palm cockatoo]]s use tools to drum.<ref>{{Cite journal|last1=Murphy |first1=Stephen |year=2003 |title=The breeding biology of palm cockatoos (''Probosciger aterrimus''): a case of a slow life history |journal=[[Journal of Zoology]] |volume=261 |issue=4 |pages=327–339 |doi=10.1017/S0952836903004175 |last2=Legge |first2=Sarah |last3=Heinsohn |first3=Robert}}</ref>


===Flocking and other associations===
===Flocking and other associations===
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[[File:Caribbean Flamingo2 (Phoenicopterus ruber) (0424) - Relic38.jpg|thumb|alt=Pink flamingo with grey legs and long neck pressed against body and head tucked under wings|Many birds, like this [[American flamingo]], tuck their head into their back when sleeping.]]
[[File:Caribbean Flamingo2 (Phoenicopterus ruber) (0424) - Relic38.jpg|thumb|alt=Pink flamingo with grey legs and long neck pressed against body and head tucked under wings|Many birds, like this [[American flamingo]], tuck their head into their back when sleeping.]]


The high metabolic rates of birds during the active part of the day is supplemented by rest at other times. Sleeping birds often use a type of sleep known as vigilant sleep, where periods of rest are interspersed with quick eye-opening "peeks", allowing them to be sensitive to disturbances and enable rapid escape from threats.<ref>{{Cite journal|last1=Gauthier-Clerc |first1=Michael |year=2000 |title=Sleep-Vigilance Trade-off in Gadwall during the Winter Period |journal=The Condor |volume=102 |issue=2 |pages=307–313 |url=http://sora.unm.edu/sites/default/files/journals/condor/v102n02/p0307-p0313.pdf |archive-url=https://web.archive.org/web/20041227194439/http://sora.unm.edu/sites/default/files/journals/condor/v102n02/p0307-p0313.pdf |archive-date=27 December 2004 |doi=10.1650/0010-5422(2000)102[0307:SVTOIG]2.0.CO;2|last2=Tamisier |first2=Alain |last3=Cézilly |first3=Frank|jstor=1369642}}</ref> [[Swift (bird)|Swift]]s are believed to be able to sleep in flight and radar observations suggest that they orient themselves to face the wind in their roosting flight.<ref>{{Cite journal|journal=The Journal of Experimental Biology|volume=205|pages=905–910|date=1 April 2002|title=Harmonic oscillatory orientation relative to the wind in nocturnal roosting flights of the swift ''Apus apus''|first=Johan|last=Bäckman|url=http://jeb.biologists.org/cgi/content/full/205/7/905|pmid=11916987|issue=7|author2=A|doi=10.1242/jeb.205.7.905}}</ref> It has been suggested that there may be certain kinds of sleep which are possible even when in flight.<ref>{{Cite journal|last=Rattenborg|first=Niels C. |year=2006 |title=Do birds sleep in flight? |journal=Die Naturwissenschaften |volume=93 |issue=9 |pages=413–425 |doi=10.1007/s00114-006-0120-3|pmid=16688436|bibcode=2006NW.....93..413R|s2cid=1736369 }}</ref>
The high metabolic rates of birds during the active part of the day is supplemented by rest at other times. Sleeping birds often use a type of sleep known as vigilant sleep, where periods of rest are interspersed with quick eye-opening "peeks", allowing them to be sensitive to disturbances and enable rapid escape from threats.<ref>{{Cite journal|last1=Gauthier-Clerc |first1=Michael |year=2000 |title=Sleep-Vigilance Trade-off in Gadwall during the Winter Period |journal=The Condor |volume=102 |issue=2 |pages=307–313 |url=http://sora.unm.edu/sites/default/files/journals/condor/v102n02/p0307-p0313.pdf |archive-url=https://web.archive.org/web/20041227194439/http://sora.unm.edu/sites/default/files/journals/condor/v102n02/p0307-p0313.pdf |archive-date=27 December 2004 |doi=10.1650/0010-5422(2000)102[0307:SVTOIG]2.0.CO;2|last2=Tamisier |first2=Alain |last3=Cézilly |first3=Frank|jstor=1369642|s2cid=15957324 }}</ref> [[Swift (bird)|Swift]]s are believed to be able to sleep in flight and radar observations suggest that they orient themselves to face the wind in their roosting flight.<ref>{{Cite journal|journal=The Journal of Experimental Biology|volume=205|pages=905–910|date=1 April 2002|title=Harmonic oscillatory orientation relative to the wind in nocturnal roosting flights of the swift ''Apus apus''|first=Johan|last=Bäckman|url=http://jeb.biologists.org/cgi/content/full/205/7/905|pmid=11916987|issue=7|author2=A|doi=10.1242/jeb.205.7.905}}</ref> It has been suggested that there may be certain kinds of sleep which are possible even when in flight.<ref>{{Cite journal|last=Rattenborg|first=Niels C. |year=2006 |title=Do birds sleep in flight? |journal=Die Naturwissenschaften |volume=93 |issue=9 |pages=413–425 |doi=10.1007/s00114-006-0120-3|pmid=16688436|bibcode=2006NW.....93..413R|s2cid=1736369 }}</ref>


Some birds have also demonstrated the capacity to fall into [[slow-wave sleep]] one [[Cerebral hemisphere|hemisphere]] of the brain at a time. The birds tend to exercise this ability depending upon its position relative to the outside of the flock. This may allow the eye opposite the sleeping hemisphere to remain vigilant for [[predator]]s by viewing the outer margins of the flock. This adaptation is also known from [[marine mammal]]s.<ref>{{Cite journal|last=Milius |first=S. |date=6 February 1999|title=Half-asleep birds choose which half dozes |journal=Science News Online |volume=155 |issue= 6|page=86 |doi=10.2307/4011301 |jstor=4011301 }}</ref> [[Communal roosting]] is common because it lowers the [[thermoregulation|loss of body heat]] and decreases the risks associated with predators.<ref>{{Cite journal|last=Beauchamp |first=Guy |year=1999 |title=The evolution of communal roosting in birds: origin and secondary losses |journal=Behavioral Ecology |volume=10 |issue=6 |pages=675–687 |doi=10.1093/beheco/10.6.675 |doi-access=free }}</ref> Roosting sites are often chosen with regard to thermoregulation and safety.<ref>{{Cite journal|last=Buttemer |first=William A.|year=1985 |title=Energy relations of winter roost-site utilization by American goldfinches (''Carduelis tristis'') |journal=[[Oecologia]] |volume=68 |issue=1 |pages=126–132 |url=http://deepblue.lib.umich.edu/bitstream/2027.42/47760/1/442_2004_Article_BF00379484.pdf |doi=10.1007/BF00379484 |pmid=28310921|bibcode=1985Oecol..68..126B |hdl=2027.42/47760|s2cid=17355506|hdl-access=free }}</ref> Unusual mobile roost sites include large herbivores on the African savanna that are used by [[oxpecker]]s.<ref>{{Cite journal|last1=Palmer|first1=Meredith S.|last2=Packer|first2=Craig|date=2018|title=Giraffe bed and breakfast: Camera traps reveal Tanzanian yellow‐billed oxpeckers roosting on their large mammalian hosts|url=https://onlinelibrary.wiley.com/doi/10.1111/aje.12505|journal=African Journal of Ecology|language=en|volume=56|issue=4|pages=882–884|doi=10.1111/aje.12505|issn=0141-6707}}</ref>
Some birds have also demonstrated the capacity to fall into [[slow-wave sleep]] one [[Cerebral hemisphere|hemisphere]] of the brain at a time. The birds tend to exercise this ability depending upon its position relative to the outside of the flock. This may allow the eye opposite the sleeping hemisphere to remain vigilant for [[predator]]s by viewing the outer margins of the flock. This adaptation is also known from [[marine mammal]]s.<ref>{{Cite journal|last=Milius |first=S. |date=6 February 1999|title=Half-asleep birds choose which half dozes |journal=Science News Online |volume=155 |issue= 6|page=86 |doi=10.2307/4011301 |jstor=4011301 }}</ref> [[Communal roosting]] is common because it lowers the [[thermoregulation|loss of body heat]] and decreases the risks associated with predators.<ref>{{Cite journal|last=Beauchamp |first=Guy |year=1999 |title=The evolution of communal roosting in birds: origin and secondary losses |journal=Behavioral Ecology |volume=10 |issue=6 |pages=675–687 |doi=10.1093/beheco/10.6.675 |doi-access=free }}</ref> Roosting sites are often chosen with regard to thermoregulation and safety.<ref>{{Cite journal|last=Buttemer |first=William A.|year=1985 |title=Energy relations of winter roost-site utilization by American goldfinches (''Carduelis tristis'') |journal=[[Oecologia]] |volume=68 |issue=1 |pages=126–132 |url=http://deepblue.lib.umich.edu/bitstream/2027.42/47760/1/442_2004_Article_BF00379484.pdf |doi=10.1007/BF00379484 |pmid=28310921|bibcode=1985Oecol..68..126B |hdl=2027.42/47760|s2cid=17355506|hdl-access=free }}</ref> Unusual mobile roost sites include large herbivores on the African savanna that are used by [[oxpecker]]s.<ref>{{Cite journal|last1=Palmer|first1=Meredith S.|last2=Packer|first2=Craig|date=2018|title=Giraffe bed and breakfast: Camera traps reveal Tanzanian yellow‐billed oxpeckers roosting on their large mammalian hosts|url=https://onlinelibrary.wiley.com/doi/10.1111/aje.12505|journal=African Journal of Ecology|language=en|volume=56|issue=4|pages=882–884|doi=10.1111/aje.12505|issn=0141-6707}}</ref>


Many sleeping birds bend their heads over their backs and tuck their [[beak|bills]] in their back feathers, although others place their beaks among their breast feathers. Many birds rest on one leg, while some may pull up their legs into their feathers, especially in cold weather. [[Passerine|Perching birds]] have a tendon-locking mechanism that helps them hold on to the perch when they are asleep. Many ground birds, such as quails and pheasants, roost in trees. A few parrots of the genus ''[[Loriculus]]'' roost hanging upside down.<ref>{{Cite journal|last=Buckley |first=F.G. |date=1 January 1968|title=Upside-down Resting by Young Green-Rumped Parrotlets (''Forpus passerinus'') |journal=The Condor |volume=70 |issue=1 |page=89 |doi=10.2307/1366517 |author2=Buckley|jstor=1366517 }}</ref> Some [[hummingbird]]s go into a nightly state of [[torpor]] accompanied with a reduction of their metabolic rates.<ref>{{Cite journal|last=Carpenter |first=F. Lynn |year=1974 |title=Torpor in an Andean Hummingbird: Its Ecological Significance |journal=Science |volume=183 |issue=4124 |pages=545–547 |doi=10.1126/science.183.4124.545 |pmid=17773043 |bibcode=1974Sci...183..545C |s2cid=42021321 }}</ref> This [[Adaptation|physiological adaptation]] shows in nearly a hundred other species, including [[owlet-nightjar]]s, [[nightjar]]s, and [[woodswallow]]s. One species, the [[common poorwill]], even enters a state of [[hibernation]].<ref>{{Cite journal|last1=McKechnie |first1=Andrew E. |year=2007 |title=Torpor in an African caprimulgid, the freckled nightjar ''Caprimulgus tristigma'' |journal=Journal of Avian Biology |volume=38 |issue=3 |pages=261–266 |doi=10.1111/j.2007.0908-8857.04116.x |last2=Ashdown |first2=Robert A.M. |last3=Christian |first3=Murray B. |last4=Brigham |first4=R. Mark}}</ref> Birds do not have sweat glands, but can lose water directly through the skin, and they may cool themselves by moving to shade, standing in water, panting, increasing their surface area, fluttering their throat or using special behaviours like [[urohidrosis]] to cool themselves.<ref>{{cite book|pages=390–396|title=Ornithology|edition=4| author1=Gill, Frank B.| author2=Prum, Richard O. |publisher=W.H. Freeman|place= New York|year=2019 }}</ref><ref>{{Cite journal |last=Cabello-Vergel |first=Julián |last2=Soriano-Redondo |first2=Andrea |last3=Villegas |first3=Auxiliadora |last4=Masero |first4=José A. |last5=Guzmán |first5=Juan M. Sánchez |last6=Gutiérrez |first6=Jorge S. |date=2021 |title=Urohidrosis as an overlooked cooling mechanism in long-legged birds |url=https://www.nature.com/articles/s41598-021-99296-8 |journal=Scientific Reports |language=en |volume=11 |issue=1 |pages=20018 |doi=10.1038/s41598-021-99296-8 |issn=2045-2322 |pmc=8501033 |pmid=34625581}}</ref>
Many sleeping birds bend their heads over their backs and tuck their [[beak|bills]] in their back feathers, although others place their beaks among their breast feathers. Many birds rest on one leg, while some may pull up their legs into their feathers, especially in cold weather. [[Passerine|Perching birds]] have a tendon-locking mechanism that helps them hold on to the perch when they are asleep. Many ground birds, such as quails and pheasants, roost in trees. A few parrots of the genus ''[[Loriculus]]'' roost hanging upside down.<ref>{{Cite journal|last=Buckley |first=F.G. |date=1 January 1968|title=Upside-down Resting by Young Green-Rumped Parrotlets (''Forpus passerinus'') |journal=The Condor |volume=70 |issue=1 |page=89 |doi=10.2307/1366517 |author2=Buckley|jstor=1366517 }}</ref> Some [[hummingbird]]s go into a nightly state of [[torpor]] accompanied with a reduction of their metabolic rates.<ref>{{Cite journal|last=Carpenter |first=F. Lynn |year=1974 |title=Torpor in an Andean Hummingbird: Its Ecological Significance |journal=Science |volume=183 |issue=4124 |pages=545–547 |doi=10.1126/science.183.4124.545 |pmid=17773043 |bibcode=1974Sci...183..545C |s2cid=42021321 }}</ref> This [[Adaptation|physiological adaptation]] shows in nearly a hundred other species, including [[owlet-nightjar]]s, [[nightjar]]s, and [[woodswallow]]s. One species, the [[common poorwill]], even enters a state of [[hibernation]].<ref>{{Cite journal|last1=McKechnie |first1=Andrew E. |year=2007 |title=Torpor in an African caprimulgid, the freckled nightjar ''Caprimulgus tristigma'' |journal=Journal of Avian Biology |volume=38 |issue=3 |pages=261–266 |doi=10.1111/j.2007.0908-8857.04116.x |last2=Ashdown |first2=Robert A.M. |last3=Christian |first3=Murray B. |last4=Brigham |first4=R. Mark}}</ref> Birds do not have sweat glands, but can lose water directly through the skin, and they may cool themselves by moving to shade, standing in water, panting, increasing their surface area, fluttering their throat or using special behaviours like [[urohidrosis]] to cool themselves.<ref>{{cite book|pages=390–396|title=Ornithology|edition=4| author1=Gill, Frank B.| author2=Prum, Richard O. |publisher=W.H. Freeman|place= New York|year=2019 }}</ref><ref>{{Cite journal |last1=Cabello-Vergel |first1=Julián |last2=Soriano-Redondo |first2=Andrea |last3=Villegas |first3=Auxiliadora |last4=Masero |first4=José A. |last5=Guzmán |first5=Juan M. Sánchez |last6=Gutiérrez |first6=Jorge S. |date=2021 |title=Urohidrosis as an overlooked cooling mechanism in long-legged birds |journal=Scientific Reports |language=en |volume=11 |issue=1 |pages=20018 |doi=10.1038/s41598-021-99296-8 |issn=2045-2322 |pmc=8501033 |pmid=34625581|bibcode=2021NatSR..1120018C }}</ref>


===Breeding===
===Breeding===
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Ninety-five per cent of bird species are socially monogamous. These species pair for at least the length of the breeding season or—in some cases—for several years or until the death of one mate.<ref>{{Cite journal|last=Freed|first=Leonard A.|year=1987|title=The Long-Term Pair Bond of Tropical House Wrens: Advantage or Constraint?|journal=[[The American Naturalist]]|volume=130|issue=4|pages=507–525|doi=10.1086/284728|s2cid=84735736}}</ref> Monogamy allows for both [[paternal care]] and [[Parental investment|biparental care]], which is especially important for species in which females require males' assistance for successful brood-rearing.<ref>{{Cite journal|last=Gowaty|first=Patricia A.|title=Male Parental Care and Apparent Monogamy among Eastern Bluebirds (''Sialia sialis'')|journal=[[The American Naturalist]]|volume=121|issue=2|pages=149–160|year=1983|doi=10.1086/284047|s2cid=84258620}}</ref> Among many socially monogamous species, [[extra-pair copulation]] (infidelity) is common.<ref>{{Cite journal|last1=Westneat|first1=David F.|year=2003|title=Extra-pair paternity in birds: Causes, correlates, and conflict|doi=10.1146/annurev.ecolsys.34.011802.132439|journal=[[Annual Review of Ecology, Evolution, and Systematics]]|volume=34|pages=365–396|last2=Stewart|first2=Ian R.K.}}</ref> Such behaviour typically occurs between dominant males and females paired with subordinate males, but may also be the result of [[forced copulation]] in ducks and other [[anatidae|anatids]].<ref>{{Cite journal|last1=Gowaty|first1=Patricia A.|year=1998|title=Ultimate causation of aggressive and forced copulation in birds: Female resistance, the CODE hypothesis, and social monogamy|journal=[[American Zoologist]]|volume=38|issue=1|pages=207–225|doi=10.1093/icb/38.1.207|last2=Buschhaus|first2=Nancy|doi-access=free}}</ref>
Ninety-five per cent of bird species are socially monogamous. These species pair for at least the length of the breeding season or—in some cases—for several years or until the death of one mate.<ref>{{Cite journal|last=Freed|first=Leonard A.|year=1987|title=The Long-Term Pair Bond of Tropical House Wrens: Advantage or Constraint?|journal=[[The American Naturalist]]|volume=130|issue=4|pages=507–525|doi=10.1086/284728|s2cid=84735736}}</ref> Monogamy allows for both [[paternal care]] and [[Parental investment|biparental care]], which is especially important for species in which females require males' assistance for successful brood-rearing.<ref>{{Cite journal|last=Gowaty|first=Patricia A.|title=Male Parental Care and Apparent Monogamy among Eastern Bluebirds (''Sialia sialis'')|journal=[[The American Naturalist]]|volume=121|issue=2|pages=149–160|year=1983|doi=10.1086/284047|s2cid=84258620}}</ref> Among many socially monogamous species, [[extra-pair copulation]] (infidelity) is common.<ref>{{Cite journal|last1=Westneat|first1=David F.|year=2003|title=Extra-pair paternity in birds: Causes, correlates, and conflict|doi=10.1146/annurev.ecolsys.34.011802.132439|journal=[[Annual Review of Ecology, Evolution, and Systematics]]|volume=34|pages=365–396|last2=Stewart|first2=Ian R.K.}}</ref> Such behaviour typically occurs between dominant males and females paired with subordinate males, but may also be the result of [[forced copulation]] in ducks and other [[anatidae|anatids]].<ref>{{Cite journal|last1=Gowaty|first1=Patricia A.|year=1998|title=Ultimate causation of aggressive and forced copulation in birds: Female resistance, the CODE hypothesis, and social monogamy|journal=[[American Zoologist]]|volume=38|issue=1|pages=207–225|doi=10.1093/icb/38.1.207|last2=Buschhaus|first2=Nancy|doi-access=free}}</ref>


For females, possible benefits of extra-pair copulation include getting better genes for her offspring and insuring against the possibility of infertility in her mate.<ref>{{Cite journal|last=Sheldon|first=B|year=1994|title=Male Phenotype, Fertility, and the Pursuit of Extra-Pair Copulations by Female Birds|journal=[[Proceedings of the Royal Society B]]|volume=257|issue=1348|pages=25–30|doi=10.1098/rspb.1994.0089|bibcode=1994RSPSB.257...25S|s2cid=85745432}}</ref> Males of species that engage in extra-pair copulations will closely guard their mates to ensure the parentage of the offspring that they raise.<ref>{{Cite journal|last1=Wei|first1=G|year=2005|title=Copulations and mate guarding of the Chinese Egret |doi=10.1675/1524-4695(2005)28[527:CAMGOT]2.0.CO;2|journal=Waterbirds|volume=28|issue=4|pages=527–530|last2=Zuo-Hua|first2=Yin|last3=Fu-Min|first3=Lei}}</ref>
For females, possible benefits of extra-pair copulation include getting better genes for her offspring and insuring against the possibility of infertility in her mate.<ref>{{Cite journal|last=Sheldon|first=B|year=1994|title=Male Phenotype, Fertility, and the Pursuit of Extra-Pair Copulations by Female Birds|journal=[[Proceedings of the Royal Society B]]|volume=257|issue=1348|pages=25–30|doi=10.1098/rspb.1994.0089|bibcode=1994RSPSB.257...25S|s2cid=85745432}}</ref> Males of species that engage in extra-pair copulations will closely guard their mates to ensure the parentage of the offspring that they raise.<ref>{{Cite journal|last1=Wei|first1=G|year=2005|title=Copulations and mate guarding of the Chinese Egret |doi=10.1675/1524-4695(2005)28[527:CAMGOT]2.0.CO;2|journal=Waterbirds|volume=28|issue=4|pages=527–530|last2=Zuo-Hua|first2=Yin|last3=Fu-Min|first3=Lei|s2cid=86336632}}</ref>


Other mating systems, including [[polygyny]], [[polyandry]], [[polygamy]], [[polygynandry]], and [[promiscuity]], also occur.<ref name = "Gill"/> Polygamous breeding systems arise when females are able to raise broods without the help of males.<ref name = "Gill"/> Mating systems vary across bird families<ref>{{Cite journal |last=Owens |first=Ian P. F. |last2=Bennett |first2=Peter M. |date=1997 |title=Variation in mating system among birds: ecological basis revealed by hierarchical comparative analysis of mate desertion |url=https://royalsocietypublishing.org/doi/10.1098/rspb.1997.0152 |journal=Proceedings of the Royal Society of London. Series B: Biological Sciences |language=en |volume=264 |issue=1385 |pages=1103–1110 |doi=10.1098/rspb.1997.0152 |issn=0962-8452 |pmc=1688567}}</ref> but variations within species are thought to be driven by environmental conditions.<ref>{{Cite journal |last=Petrie |first=Marion |last2=Kempenaers |first2=Bart |date=1998 |title=Extra-pair paternity in birds: explaining variation between species and populations |url=https://linkinghub.elsevier.com/retrieve/pii/S0169534797012329 |journal=Trends in Ecology & Evolution |language=en |volume=13 |issue=2 |pages=52–58 |doi=10.1016/S0169-5347(97)01232-9}}</ref>
Other mating systems, including [[polygyny]], [[polyandry]], [[polygamy]], [[polygynandry]], and [[promiscuity]], also occur.<ref name = "Gill"/> Polygamous breeding systems arise when females are able to raise broods without the help of males.<ref name = "Gill"/> Mating systems vary across bird families<ref>{{Cite journal |last1=Owens |first1=Ian P. F. |last2=Bennett |first2=Peter M. |date=1997 |title=Variation in mating system among birds: ecological basis revealed by hierarchical comparative analysis of mate desertion |journal=Proceedings of the Royal Society of London. Series B: Biological Sciences |language=en |volume=264 |issue=1385 |pages=1103–1110 |doi=10.1098/rspb.1997.0152 |issn=0962-8452 |pmc=1688567}}</ref> but variations within species are thought to be driven by environmental conditions.<ref>{{Cite journal |last1=Petrie |first1=Marion |last2=Kempenaers |first2=Bart |date=1998 |title=Extra-pair paternity in birds: explaining variation between species and populations |url=https://linkinghub.elsevier.com/retrieve/pii/S0169534797012329 |journal=Trends in Ecology & Evolution |language=en |volume=13 |issue=2 |pages=52–58 |doi=10.1016/S0169-5347(97)01232-9|pmid=21238200 }}</ref>


Breeding usually involves some form of courtship display, typically performed by the male.<ref>{{Cite book|last=Short|first=Lester L.|year=1993|title=Birds of the World and their Behavior|publisher=Henry Holt and Co|location=New York|isbn=0-8050-1952-9|url=https://archive.org/details/livesofbirdsbird00shor}}</ref> Most displays are rather simple and involve some type of [[bird vocalization|song]]. Some displays, however, are quite elaborate. Depending on the species, these may include wing or tail drumming, dancing, aerial flights, or communal [[Lek mating|lekking]]. Females are generally the ones that drive partner selection,<ref>{{Cite book|last=Burton|first=R|year=1985|title=Bird Behavior|publisher=Alfred A. Knopf, Inc|isbn=0-394-53957-5|url=https://archive.org/details/birdbehavior0000burt}}</ref> although in the polyandrous [[phalaropes]], this is reversed: plainer males choose brightly coloured females.<ref>{{Cite journal|last1=Schamel|first1=D|year=2004|title=Mate guarding, copulation strategies and paternity in the sex-role reversed, socially polyandrous red-necked phalarope ''Phalaropus lobatus''|journal=Behavioral Ecology and Sociobiology|volume=57|issue=2|pages=110–118|doi=10.1007/s00265-004-0825-2|last2=Tracy|first2=Diane M.|last3=Lank|first3=David B.|last4=Westneat|first4=David F.|s2cid=26038182}}</ref> [[Courtship feeding]], [[Billing (birds)|billing]] and {{Birdgloss|allopreening}} are commonly performed between partners, generally after the birds have paired and mated.<ref name="Attenborough">{{Cite book|last=Attenborough |first=David |author-link=David Attenborough |year=1998 |title=The Life of Birds |location=Princeton |publisher=Princeton University Press |isbn=0-691-01633-X|title-link=The Life of Birds }}</ref>
Breeding usually involves some form of courtship display, typically performed by the male.<ref>{{Cite book|last=Short|first=Lester L.|year=1993|title=Birds of the World and their Behavior|publisher=Henry Holt and Co|location=New York|isbn=0-8050-1952-9|url=https://archive.org/details/livesofbirdsbird00shor}}</ref> Most displays are rather simple and involve some type of [[bird vocalization|song]]. Some displays, however, are quite elaborate. Depending on the species, these may include wing or tail drumming, dancing, aerial flights, or communal [[Lek mating|lekking]]. Females are generally the ones that drive partner selection,<ref>{{Cite book|last=Burton|first=R|year=1985|title=Bird Behavior|publisher=Alfred A. Knopf, Inc|isbn=0-394-53957-5|url=https://archive.org/details/birdbehavior0000burt}}</ref> although in the polyandrous [[phalaropes]], this is reversed: plainer males choose brightly coloured females.<ref>{{Cite journal|last1=Schamel|first1=D|year=2004|title=Mate guarding, copulation strategies and paternity in the sex-role reversed, socially polyandrous red-necked phalarope ''Phalaropus lobatus''|journal=Behavioral Ecology and Sociobiology|volume=57|issue=2|pages=110–118|doi=10.1007/s00265-004-0825-2|last2=Tracy|first2=Diane M.|last3=Lank|first3=David B.|last4=Westneat|first4=David F.|s2cid=26038182}}</ref> [[Courtship feeding]], [[Billing (birds)|billing]] and {{Birdgloss|allopreening}} are commonly performed between partners, generally after the birds have paired and mated.<ref name="Attenborough">{{Cite book|last=Attenborough |first=David |author-link=David Attenborough |year=1998 |title=The Life of Birds |location=Princeton |publisher=Princeton University Press |isbn=0-691-01633-X|title-link=The Life of Birds }}</ref>
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==Ecology==
==Ecology==
[[File:Pinzón azul de Gran Canaria (macho), M. A. Peña.jpg|thumb|right|[[Gran Canaria blue chaffinch]], an example of a bird highly specialised in its habitat, in this case in the [[Pinus canariensis|Canarian pine]] forests]]
[[File:Pinzón azul de Gran Canaria (macho), M. A. Peña.jpg|thumb|right|[[Gran Canaria blue chaffinch]], an example of a bird highly specialised in its habitat, in this case in the [[Pinus canariensis|Canarian pine]] forests]]
Birds occupy a wide range of ecological positions.<ref name = "flycatcher"/> While some birds are generalists, others are highly specialised in their habitat or food requirements. Even within a single habitat, such as a forest, the [[Ecological niche|niches]] occupied by different species of birds vary, with some species feeding in the [[forest canopy]], others beneath the canopy, and still others on the forest floor. Forest birds may be [[insectivore]]s, [[frugivore]]s, or [[nectarivore]]s. Aquatic birds generally feed by fishing, plant eating, and piracy or [[kleptoparasitism]]. Many grassland birds are granivores. Birds of prey specialise in hunting mammals or other birds, while vultures are specialised [[scavenger]]s. Birds are also preyed upon by a range of mammals including a few [[Avivore|avivorous]] bats.<ref>{{Cite journal |last=Gong |first=Lixin |last2=Shi |first2=Biye |last3=Wu |first3=Hui |last4=Feng |first4=Jiang |last5=Jiang |first5=Tinglei |date=2021 |title=Who’s for dinner? Bird prey diversity and choice in the great evening bat, Ia io |url=https://onlinelibrary.wiley.com/doi/10.1002/ece3.7667 |journal=Ecology and Evolution |language=en |volume=11 |issue=13 |pages=8400–8409 |doi=10.1002/ece3.7667 |issn=2045-7758 |pmc=8258197 |pmid=34257905}}</ref> A wide range of endo- and ectoparasites depend on birds and some parasites that are transmitted from parent to young have [[Coevolution|co-evolved]] and show host-specificity.<ref>{{Cite journal |last=Križanauskienė |first=Asta |last2=Hellgren |first2=Olof |last3=Kosarev |first3=Vladislav |last4=Sokolov |first4=Leonid |last5=Bensch |first5=Staffan |last6=Valkiūnas |first6=Gediminas |date=2006 |title=Variation in host specificty between species of avian hemosporidian parasites: evidence from parasite morphology and cytochrome b gene sequences |url=http://www.bioone.org/doi/abs/10.1645/GE-873R.1 |journal=Journal of Parasitology |language=en |volume=92 |issue=6 |pages=1319–1324 |doi=10.1645/GE-873R.1 |issn=0022-3395}}</ref><ref>{{Cite journal |last=John |first=J |date=1995 |title=Parasites and the avian spleen: helminths |url=https://linkinghub.elsevier.com/retrieve/pii/0024406695900381 |journal=Biological Journal of the Linnean Society |language=en |volume=54 |issue=1 |pages=87–106 |doi=10.1016/0024-4066(95)90038-1}}</ref>
Birds occupy a wide range of ecological positions.<ref name = "flycatcher"/> While some birds are generalists, others are highly specialised in their habitat or food requirements. Even within a single habitat, such as a forest, the [[Ecological niche|niches]] occupied by different species of birds vary, with some species feeding in the [[forest canopy]], others beneath the canopy, and still others on the forest floor. Forest birds may be [[insectivore]]s, [[frugivore]]s, or [[nectarivore]]s. Aquatic birds generally feed by fishing, plant eating, and piracy or [[kleptoparasitism]]. Many grassland birds are granivores. Birds of prey specialise in hunting mammals or other birds, while vultures are specialised [[scavenger]]s. Birds are also preyed upon by a range of mammals including a few [[Avivore|avivorous]] bats.<ref>{{Cite journal |last1=Gong |first1=Lixin |last2=Shi |first2=Biye |last3=Wu |first3=Hui |last4=Feng |first4=Jiang |last5=Jiang |first5=Tinglei |date=2021 |title=Who's for dinner? Bird prey diversity and choice in the great evening bat, Ia io |journal=Ecology and Evolution |language=en |volume=11 |issue=13 |pages=8400–8409 |doi=10.1002/ece3.7667 |issn=2045-7758 |pmc=8258197 |pmid=34257905}}</ref> A wide range of endo- and ectoparasites depend on birds and some parasites that are transmitted from parent to young have [[Coevolution|co-evolved]] and show host-specificity.<ref>{{Cite journal |last1=Križanauskienė |first1=Asta |last2=Hellgren |first2=Olof |last3=Kosarev |first3=Vladislav |last4=Sokolov |first4=Leonid |last5=Bensch |first5=Staffan |last6=Valkiūnas |first6=Gediminas |date=2006 |title=Variation in host specificty between species of avian hemosporidian parasites: evidence from parasite morphology and cytochrome b gene sequences |url=http://www.bioone.org/doi/abs/10.1645/GE-873R.1 |journal=Journal of Parasitology |language=en |volume=92 |issue=6 |pages=1319–1324 |doi=10.1645/GE-873R.1 |pmid=17304814 |s2cid=27746219 |issn=0022-3395}}</ref><ref>{{Cite journal |last=John |first=J |date=1995 |title=Parasites and the avian spleen: helminths |url=https://linkinghub.elsevier.com/retrieve/pii/0024406695900381 |journal=Biological Journal of the Linnean Society |language=en |volume=54 |issue=1 |pages=87–106 |doi=10.1016/0024-4066(95)90038-1}}</ref>


Some nectar-feeding birds are important pollinators, and many frugivores play a key role in seed dispersal.<ref name = "Clout">{{cite journal | last1 = Clout | first1 = M | last2 = Hay | first2 = J | year = 1989 | title = The importance of birds as browsers, pollinators and seed dispersers in New Zealand forests | url = http://www.newzealandecology.org/nzje/free_issues/NZJEcol12_s_27.pdf | journal = New Zealand Journal of Ecology | volume = 12 | pages = 27–33 }}</ref> Plants and pollinating birds often [[coevolution|coevolve]],<ref>{{cite journal | last1=Gary Stiles | first1=F. | title=Geographical Aspects of Bird-Flower Coevolution, with Particular Reference to Central America | journal=Annals of the Missouri Botanical Garden | volume=68 | issue=2 | pages=323–351 | year=1981 |doi= 10.2307/2398801 | jstor=2398801| url=https://www.biodiversitylibrary.org/part/38387 }}</ref> and in some cases a flower's primary pollinator is the only species capable of reaching its nectar.<ref>{{cite journal | last1 = Temeles | first1 = E | last2 = Linhart | first2 = Y | last3 = Masonjones | first3 = M | last4 = Masonjones | first4 = H | year = 2002 | title = The Role of Flower Width in Hummingbird Bill Length–Flower Length Relationships | url = http://www.amherst.edu/~ejtemeles/Temeles%20et%20al%202002%20biotropica.pdf | journal = Biotropica | volume = 34 | issue = 1| pages = 68–80 | doi=10.1111/j.1744-7429.2002.tb00243.x}}</ref>
Some nectar-feeding birds are important pollinators, and many frugivores play a key role in seed dispersal.<ref name = "Clout">{{cite journal | last1 = Clout | first1 = M | last2 = Hay | first2 = J | year = 1989 | title = The importance of birds as browsers, pollinators and seed dispersers in New Zealand forests | url = http://www.newzealandecology.org/nzje/free_issues/NZJEcol12_s_27.pdf | journal = New Zealand Journal of Ecology | volume = 12 | pages = 27–33 }}</ref> Plants and pollinating birds often [[coevolution|coevolve]],<ref>{{cite journal | last1=Gary Stiles | first1=F. | title=Geographical Aspects of Bird-Flower Coevolution, with Particular Reference to Central America | journal=Annals of the Missouri Botanical Garden | volume=68 | issue=2 | pages=323–351 | year=1981 |doi= 10.2307/2398801 | jstor=2398801| url=https://www.biodiversitylibrary.org/part/38387 }}</ref> and in some cases a flower's primary pollinator is the only species capable of reaching its nectar.<ref>{{cite journal | last1 = Temeles | first1 = E | last2 = Linhart | first2 = Y | last3 = Masonjones | first3 = M | last4 = Masonjones | first4 = H | year = 2002 | title = The Role of Flower Width in Hummingbird Bill Length–Flower Length Relationships | url = http://www.amherst.edu/~ejtemeles/Temeles%20et%20al%202002%20biotropica.pdf | journal = Biotropica | volume = 34 | issue = 1| pages = 68–80 | doi=10.1111/j.1744-7429.2002.tb00243.x| s2cid = 16315843 }}</ref>


Birds are often important to island ecology. Birds have frequently reached islands that mammals have not; on those islands, birds may fulfil ecological roles typically played by larger animals. For example, in New Zealand nine species of [[moa]] were important browsers, as are the [[kererū]] and [[kokako]] today.<ref name = "Clout"/> Today the plants of New Zealand retain the defensive adaptations evolved to protect them from the extinct moa.<ref>{{cite journal | last1=Bond | first1=William J. | last2=Lee | first2=William G. | last3=Craine | first3=Joseph M. | title=Plant structural defences against browsing birds: a legacy of New Zealand's extinct moas | journal=Oikos | volume=104 | pages=500–508 | year=2004 | doi = 10.1111/j.0030-1299.2004.12720.x | issue=3}}</ref>
Birds are often important to island ecology. Birds have frequently reached islands that mammals have not; on those islands, birds may fulfil ecological roles typically played by larger animals. For example, in New Zealand nine species of [[moa]] were important browsers, as are the [[kererū]] and [[kokako]] today.<ref name = "Clout"/> Today the plants of New Zealand retain the defensive adaptations evolved to protect them from the extinct moa.<ref>{{cite journal | last1=Bond | first1=William J. | last2=Lee | first2=William G. | last3=Craine | first3=Joseph M. | title=Plant structural defences against browsing birds: a legacy of New Zealand's extinct moas | journal=Oikos | volume=104 | pages=500–508 | year=2004 | doi = 10.1111/j.0030-1299.2004.12720.x | issue=3}}</ref>
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Many birds act as [[ecosystem engineer]]s through the construction of nests, which provide important microhabitats and food for hundreds of species of invertebrates.<ref>{{Cite journal|last1=Berner|first1=Lewis|last2=Hicks|first2=Ellis A.|date=June 1959|title=Checklist and Bibliography on the Occurrence of Insects in Birds Nests|url=http://dx.doi.org/10.2307/3492142|journal=The Florida Entomologist|volume=42|issue=2|pages=92|doi=10.2307/3492142|jstor=3492142|issn=0015-4040}}</ref><ref>{{Cite journal|last1=Boyes|first1=Douglas H.|last2=Lewis|first2=Owen T.|date=2019|title=Ecology of Lepidoptera associated with bird nests in mid-Wales, UK|url=https://onlinelibrary.wiley.com/doi/abs/10.1111/een.12669|journal=Ecological Entomology|language=en|volume=44|issue=1|pages=1–10|doi=10.1111/een.12669|s2cid=91557693|issn=1365-2311}}</ref> Nesting [[seabird]]s may affect the ecology of islands and surrounding seas, principally through the concentration of large quantities of [[guano]], which may enrich the local soil<ref>{{cite journal | last1 = Wainright | first1 = S | last2 = Haney | first2 = J | last3 = Kerr | first3 = C | last4 = Golovkin | first4 = A | last5 = Flint | first5 = M | year = 1998 | title = Utilization of nitrogen derived from seabird guano by terrestrial and marine plants at St. Paul, Pribilof Islands, Bering Sea, Alaska | journal = Marine Ecology | volume = 131 | issue = 1| pages = 63–71 | doi=10.1007/s002270050297| s2cid = 83734364 }}</ref> and the surrounding seas.<ref>{{cite journal | doi = 10.3354/meps032247 | last1 = Bosman | first1 = A | last2 = Hockey | first2 = A | year = 1986 | title = Seabird guano as a determinant of rocky intertidal community structure | journal = Marine Ecology Progress Series | volume = 32 |pages = 247–257 | bibcode = 1986MEPS...32..247B | doi-access = free }}</ref>
Many birds act as [[ecosystem engineer]]s through the construction of nests, which provide important microhabitats and food for hundreds of species of invertebrates.<ref>{{Cite journal|last1=Berner|first1=Lewis|last2=Hicks|first2=Ellis A.|date=June 1959|title=Checklist and Bibliography on the Occurrence of Insects in Birds Nests|url=http://dx.doi.org/10.2307/3492142|journal=The Florida Entomologist|volume=42|issue=2|pages=92|doi=10.2307/3492142|jstor=3492142|issn=0015-4040}}</ref><ref>{{Cite journal|last1=Boyes|first1=Douglas H.|last2=Lewis|first2=Owen T.|date=2019|title=Ecology of Lepidoptera associated with bird nests in mid-Wales, UK|url=https://onlinelibrary.wiley.com/doi/abs/10.1111/een.12669|journal=Ecological Entomology|language=en|volume=44|issue=1|pages=1–10|doi=10.1111/een.12669|s2cid=91557693|issn=1365-2311}}</ref> Nesting [[seabird]]s may affect the ecology of islands and surrounding seas, principally through the concentration of large quantities of [[guano]], which may enrich the local soil<ref>{{cite journal | last1 = Wainright | first1 = S | last2 = Haney | first2 = J | last3 = Kerr | first3 = C | last4 = Golovkin | first4 = A | last5 = Flint | first5 = M | year = 1998 | title = Utilization of nitrogen derived from seabird guano by terrestrial and marine plants at St. Paul, Pribilof Islands, Bering Sea, Alaska | journal = Marine Ecology | volume = 131 | issue = 1| pages = 63–71 | doi=10.1007/s002270050297| s2cid = 83734364 }}</ref> and the surrounding seas.<ref>{{cite journal | doi = 10.3354/meps032247 | last1 = Bosman | first1 = A | last2 = Hockey | first2 = A | year = 1986 | title = Seabird guano as a determinant of rocky intertidal community structure | journal = Marine Ecology Progress Series | volume = 32 |pages = 247–257 | bibcode = 1986MEPS...32..247B | doi-access = free }}</ref>


A wide variety of [[avian ecology field methods]], including counts, nest monitoring, and capturing and marking, are used for researching avian ecology.<ref>{{cite book |author2=Newton, Ian |author1=Sutherland, William J. |title=Bird Ecology and Conservation. A Handbook of Techniques |author3=Green, Rhys E. |publisher=Oxford University Press |isbn=0198520859}}</ref>
A wide variety of [[avian ecology field methods]], including counts, nest monitoring, and capturing and marking, are used for researching avian ecology.<ref>{{cite book |author2=Newton, Ian |author1=Sutherland, William J. |title=Bird Ecology and Conservation. A Handbook of Techniques |author3=Green, Rhys E. |year=2004 |publisher=Oxford University Press |isbn=0198520859}}</ref>


==Relationship with humans==
==Relationship with humans==
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[[File:California-Condor3-Szmurlo edit.jpg|thumb|right|upright|alt=Large black bird with featherless head and hooked bill|The [[California condor]] once numbered only 22 birds, but conservation measures have raised that to over 500 today.]]
[[File:California-Condor3-Szmurlo edit.jpg|thumb|right|upright|alt=Large black bird with featherless head and hooked bill|The [[California condor]] once numbered only 22 birds, but conservation measures have raised that to over 500 today.]]


Although human activities have allowed the expansion of a few species, such as the [[barn swallow]] and [[European starling]], they have caused population decreases or [[extinction]] in many other species. Over a hundred bird species have gone extinct in historical times,<ref>Fuller, Errol (2000). ''Extinct Birds'' (2nd ed.). [[Oxford University Press]], Oxford & New York. {{ISBN|0-19-850837-9}}</ref> although the most dramatic human-caused avian extinctions, eradicating an estimated 750–1800 species, occurred during the human colonisation of [[Melanesia]]n, [[Polynesia]]n, and [[Micronesia]]n islands.<ref>Steadman, D. (2006). ''Extinction and Biogeography in Tropical Pacific Birds'', University of Chicago Press. {{ISBN|978-0-226-77142-7}}</ref> Many bird populations are declining worldwide, with 1,227 species listed as [[threatened species|threatened]] by [[BirdLife International]] and the [[IUCN]] in 2009.<ref>{{cite web |title=BirdLife International announces more Critically Endangered birds than ever before |publisher=[[BirdLife International]] |date=14 May 2009 |url=http://www.birdlife.org/news/pr/2009/05/red_list.html |access-date=15 May 2009 |archive-date=20 June 2013 |archive-url=https://www.webcitation.org/6HWarDrE8?url=http://www.birdlife.org/news/pr/2009/05/red_list.html |url-status=dead }}</ref><ref>{{cite news|url=http://news.bbc.co.uk/2/hi/science/nature/8045971.stm |first=Mark |last=Kinver |title=Birds at risk reach record high |work=BBC News Online |date=13 May 2009 |access-date=15 May 2009}}</ref>
Although human activities have allowed the expansion of a few species, such as the [[barn swallow]] and [[European starling]], they have caused population decreases or [[extinction]] in many other species. Over a hundred bird species have gone extinct in historical times,<ref>Fuller, Errol (2000). ''Extinct Birds'' (2nd ed.). [[Oxford University Press]], Oxford & New York. {{ISBN|0-19-850837-9}}</ref> although the most dramatic human-caused avian extinctions, eradicating an estimated 750–1800 species, occurred during the human colonisation of [[Melanesia]]n, [[Polynesia]]n, and [[Micronesia]]n islands.<ref>Steadman, D. (2006). ''Extinction and Biogeography in Tropical Pacific Birds'', University of Chicago Press. {{ISBN|978-0-226-77142-7}}</ref> Many bird populations are declining worldwide, with 1,227 species listed as [[threatened species|threatened]] by [[BirdLife International]] and the [[IUCN]] in 2009.<ref>{{cite web |title=BirdLife International announces more Critically Endangered birds than ever before |publisher=[[BirdLife International]] |date=14 May 2009 |url=http://www.birdlife.org/news/pr/2009/05/red_list.html |access-date=15 May 2009 |archive-date=17 June 2013 |archive-url=https://web.archive.org/web/20130617183344/http://www.birdlife.org/news/pr/2009/05/red_list.html |url-status=dead }}</ref><ref>{{cite news|url=http://news.bbc.co.uk/2/hi/science/nature/8045971.stm |first=Mark |last=Kinver |title=Birds at risk reach record high |work=BBC News Online |date=13 May 2009 |access-date=15 May 2009}}</ref>


The most commonly cited human threat to birds is [[Habitat destruction|habitat loss]].<ref>Norris K, Pain D (eds, 2002). ''Conserving Bird Biodiversity: General Principles and their Application'' Cambridge University Press. {{ISBN|978-0-521-78949-3}}</ref> Other threats include overhunting, accidental mortality due to collisions with [[bird-skyscraper collisions|buildings]] or [[bird strike|vehicles]], [[long-line fishing]] [[bycatch]],<ref>{{Cite journal | doi=10.1016/0006-3207(91)90031-4 | last1=Brothers | first1=N.P. | year=1991 | title=Albatross mortality and associated bait loss in the Japanese longline fishery in the southern ocean |  journal=Biological Conservation | volume=55 | issue=3| pages=255–268 }}</ref> pollution (including [[oil spill]]s and pesticide use),<ref>{{cite journal | last1=Wurster | first1=D. | last2=Wurster | first2=C. | last3=Strickland | first3=W. | date=July 1965 | title=Bird Mortality Following DDT Spray for Dutch Elm Disease |  journal=Ecology | volume=46 | issue=4| pages=488–499 | doi =10.2307/1934880 | jstor=1934880 }}; {{cite journal | doi=10.1126/science.148.3666.90 | title=Bird Mortality after Spraying for Dutch Elm Disease with DDT | year=1965 | last1=Wurster | first1=C.F. | last2=Wurster | first2=D.H. | last3=Strickland | first3=W.N. | journal=Science | volume=148 | issue=3666 | pages=90–91 | pmid=14258730 | bibcode=1965Sci...148...90W | s2cid=26320497 }}</ref> competition and predation from nonnative [[invasive species]],<ref>{{cite journal | last1=Blackburn | first1=T | last2=Cassey | first2=P | last3=Duncan | first3=R | last4=Evans | first4=K | last5=Gaston | first5=K | date=24 September 2004 | title=Avian Extinction and Mammalian Introductions on Oceanic Islands |  journal=[[Science (journal)|Science]] | volume=305 | issue=5692| pages=1955–1958 | doi=10.1126/science.1101617 | pmid=15448269| bibcode=2004Sci...305.1955B | s2cid=31211118 }}</ref> and climate change.
The most commonly cited human threat to birds is [[Habitat destruction|habitat loss]].<ref>Norris K, Pain D (eds, 2002). ''Conserving Bird Biodiversity: General Principles and their Application'' Cambridge University Press. {{ISBN|978-0-521-78949-3}}</ref> Other threats include overhunting, accidental mortality due to collisions with [[bird-skyscraper collisions|buildings]] or [[bird strike|vehicles]], [[long-line fishing]] [[bycatch]],<ref>{{Cite journal | doi=10.1016/0006-3207(91)90031-4 | last1=Brothers | first1=N.P. | year=1991 | title=Albatross mortality and associated bait loss in the Japanese longline fishery in the southern ocean |  journal=Biological Conservation | volume=55 | issue=3| pages=255–268 }}</ref> pollution (including [[oil spill]]s and pesticide use),<ref>{{cite journal | last1=Wurster | first1=D. | last2=Wurster | first2=C. | last3=Strickland | first3=W. | date=July 1965 | title=Bird Mortality Following DDT Spray for Dutch Elm Disease |  journal=Ecology | volume=46 | issue=4| pages=488–499 | doi =10.2307/1934880 | jstor=1934880 }}; {{cite journal | doi=10.1126/science.148.3666.90 | title=Bird Mortality after Spraying for Dutch Elm Disease with DDT | year=1965 | last1=Wurster | first1=C.F. | last2=Wurster | first2=D.H. | last3=Strickland | first3=W.N. | journal=Science | volume=148 | issue=3666 | pages=90–91 | pmid=14258730 | bibcode=1965Sci...148...90W | s2cid=26320497 }}</ref> competition and predation from nonnative [[invasive species]],<ref>{{cite journal | last1=Blackburn | first1=T | last2=Cassey | first2=P | last3=Duncan | first3=R | last4=Evans | first4=K | last5=Gaston | first5=K | date=24 September 2004 | title=Avian Extinction and Mammalian Introductions on Oceanic Islands |  journal=[[Science (journal)|Science]] | volume=305 | issue=5692| pages=1955–1958 | doi=10.1126/science.1101617 | pmid=15448269| bibcode=2004Sci...305.1955B | s2cid=31211118 }}</ref> and [[Climate change and birds|climate change]].


Governments and [[conservation biology|conservation]] groups work to protect birds, either by passing laws that [[In-situ conservation|preserve]] and [[ecological restoration|restore]] bird habitat or by establishing [[Ex-situ conservation|captive populations]] for reintroductions. Such projects have produced some successes; one study estimated that conservation efforts saved 16 species of bird that would otherwise have gone extinct between 1994 and 2004, including the [[California condor]] and [[Norfolk parakeet]].<ref>{{cite journal | doi=10.1017/S0030605306000950 | last1=Butchart | first1=S. | last2=Stattersfield | first2=A. | last3=Collar | first3=N | year=2006 | title=How many bird extinctions have we prevented? | url=http://www.birdlife.org/news/news/2006/08/butchart_et_al_2006.pdf | journal=Oryx | volume=40 | issue=3 | pages=266–79 | doi-access=free | access-date=24 May 2007 | archive-date=10 August 2011 | archive-url=https://web.archive.org/web/20110810080436/http://www.birdlife.org/news/news/2006/08/butchart_et_al_2006.pdf | url-status=dead }}</ref>
Governments and [[conservation biology|conservation]] groups work to protect birds, either by passing laws that [[In-situ conservation|preserve]] and [[ecological restoration|restore]] bird habitat or by establishing [[Ex-situ conservation|captive populations]] for reintroductions. Such projects have produced some successes; one study estimated that conservation efforts saved 16 species of bird that would otherwise have gone extinct between 1994 and 2004, including the [[California condor]] and [[Norfolk parakeet]].<ref>{{cite journal | doi=10.1017/S0030605306000950 | last1=Butchart | first1=S. | last2=Stattersfield | first2=A. | last3=Collar | first3=N | year=2006 | title=How many bird extinctions have we prevented? | journal=Oryx | volume=40 | issue=3 | pages=266–79 | doi-access=free }}</ref>


==See also==
==See also==
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* [[Avian sleep]]
* [[Avian sleep]]
* [[Bat]]
* [[Bat]]
* [[Climate change and birds]]
* [[Glossary of bird terms]]
* [[Glossary of bird terms]]
* [[Ornithology]]
* [[Ornithology]]
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