Editing Bird (section)

==Evolution and classification==
{{Main|Evolution of birds}}
[[File:Archaeopteryx_lithographica_(Berlin_specimen).jpg|alt= Slab of stone with fossil bones and feather impressions|thumb|''[[Archaeopteryx]]'' is often considered the oldest known true bird.]]
The first [[biological classification|classification]] of birds was developed by [[Francis Willughby]] and [[John Ray]] in their 1676 volume ''Ornithologiae''.<ref>{{Cite book|last=del Hoyo |first=Josep |author2=Andy Elliott|author3=Jordi Sargatal |title=Handbook of Birds of the World, Volume 1: Ostrich to Ducks |year=1992 |publisher=[[Lynx Edicions]] |location=Barcelona |isbn=84-87334-10-5}}</ref>
[[Carl Linnaeus]] modified that work in 1758 to devise the [[taxonomic classification]] system currently in use.<ref>
{{Cite book|last=Linnaeus |first=Carolus |author-link=Carl Linnaeus |title=Systema naturae per regna tria naturae, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata |publisher=Holmiae. (Laurentii Salvii) |year=1758 |page=824 |language=la|title-link=Systema Naturae }}</ref> Birds are categorised as the [[Class (biology)|biological class]] Aves in [[Linnaean taxonomy]]. [[Phylogenetic taxonomy]] places Aves in the [[clade]] [[Theropoda]] as an infraclass<ref name="Theropoda">{{Cite journal |date=January 2007 | title=Higher-order phylogeny of modern birds (Theropoda, Aves: Neornithes) based on comparative anatomy. II. Analysis and discussion | journal=[[Zoological Journal of the Linnean Society]] | volume=149 | issue=1 | pages=1–95 | doi=10.1111/j.1096-3642.2006.00293.x | pmid=18784798 | last1=Livezey | first1=Bradley C. | last2=Zusi | first2=RL | pmc=2517308}}</ref> or more recently a subclass<ref>{{Cite journal |last1=Ruggiero |first1=Michael A. |last2=Gordon |first2=Dennis P. |last3=Orrell |first3=Thomas M. |last4=Bailly |first4=Nicolas |last5=Bourgoin |first5=Thierry |last6=Brusca |first6=Richard C. |last7=Cavalier-Smith |first7=Thomas |last8=Guiry |first8=Michael D. |last9=Kirk |first9=Paul M. |date=2015-06-11 |title=Correction: A Higher Level Classification of All Living Organisms |journal=PLOS ONE |language=en |volume=10 |issue=6 |pages=e0130114 |doi=10.1371/journal.pone.0130114 |doi-access=free |pmc=5159126 |pmid=26068874|bibcode=2015PLoSO..1030114R }}</ref> or class.

===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=[[Philip J. Currie]]|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 name =Gauthier1986 >{{Cite book |last=Gauthier |first=Jacques |year=1986 |chapter=Saurischian monophyly and the origin of birds |editor=Padian, Kevin |title=The Origin of Birds and the Evolution of Flight |series=Memoirs of the California Academy of Science |volume=8 |pages=1–55 |isbn=0-940228-14-9 |publisher=Published by California Academy of Sciences |location=San Francisco, CA|url=https://biostor.org/reference/110202}}</ref> on the principle that a clade based on extant species should be limited to those extant species and their closest extinct relatives.<ref name =Gauthier1986 />

Gauthier and de Queiroz identified four different definitions for the same biological name "Aves", which is a problem.<ref name="gauthier&dequeiroz2001"/> 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,<ref name="gauthier&dequeiroz2001"/> which corresponds to meaning number 4 below. They assigned other names to the other groups.<ref name="gauthier&dequeiroz2001"/>

{{Cladogram
 |caption=The birds' phylogenetic relationships to major living reptile groups. {{small|(The [[turtle]]s' position is uncertain: Some authorities embed them inside the [[Archosaur]]s, with '''birds''' and [[crocodile]]s.)}}
 |clades={{clade
  |style=font-size:75%
  |label1=[[Reptile]]s
  |1= {{clade
     |label1=
     |1= {{clade
        |label1=[[Squamate]]s
        |1=  [[Lizard]]s & [[snake]]s
     }}
    |2= {{clade
      |label1=[[Pantestudines]]
      |1=  [[Turtle]]s
      |label2=[[Archosaur]]s
      |2= {{clade
          |label1=
          |1=  [[Crocodile]]s
          |2=  '''Birds'''
       }}
     }}
   }}
 }}
}}

# Aves can mean all [[archosaur]]s closer to birds than to [[crocodile]]s (alternately [[Avemetatarsalia]])
# Aves can mean those advanced archosaurs with feathers (alternately [[Avifilopluma]])
# Aves can mean those feathered dinosaurs that fly (alternately [[Avialae]])
# Aves can mean the last common ancestor of all the currently living birds and all of its descendants (a "[[crown group]]", in this sense synonymous with '''Neornithes''')

Under the fourth definition ''Archaeopteryx'', traditionally considered one of the earliest members of Aves, is removed from this group, becoming a non-avian dinosaur instead. These proposals have been adopted by many researchers in the field of palaeontology and [[bird evolution]], though the exact definitions applied have been inconsistent. Avialae, initially proposed to replace the traditional fossil content of Aves, is often used synonymously with the vernacular term "bird" by these researchers.<ref name=Nature>{{Cite journal|author1=Godefroit, Pascal |author2=Andrea Cau |author3=Hu Dong-Yu |author4=François Escuillié |author5=Wu Wenhao |author6=Gareth Dyke |year=2013 |title=A Jurassic avialan dinosaur from China resolves the early phylogenetic history of birds |journal=Nature |volume= 498|issue= 7454|pages= 359–362|doi=10.1038/nature12168 |pmid=23719374|bibcode=2013Natur.498..359G |author1-link=Pascal Godefroit }}</ref>

{{Cladogram|caption=Cladogram showing the results of a phylogenetic study by Cau, 2018.<ref name="paleoitalia"/>|clades=
{{clade
   | style=font-size:75%;line-height:80%
   |label1=Maniraptoromorpha
   |1={{clade
      |label1 =
      |1={{clade
         |label1 =
         |1= {{extinct}}''[[Coelurus]]''
         }}
      |2={{clade
         |1= {{extinct}}''[[Ornitholestes]]''
         |label2=[[Maniraptoriformes]]
         |2={{clade
            |1= {{extinct}}[[Ornithomimosauria]]
            |label2=[[Maniraptora]]
            |2={{clade
               |1= {{extinct}}[[Alvarezsauridae]]
               |label2=[[Pennaraptora]]
               |2={{clade
                  |label1 =
                  |1= {{extinct}}[[Oviraptorosauria]]
                  |2=  [[Paraves]]
                  }}
               }}
            }}
         }}
      }}
   }}
}}

Most researchers define Avialae as branch-based clade, though definitions vary. Many authors have used a definition similar to "all [[theropod]]s closer to birds than to ''[[Deinonychus]]''",<ref name="weishampel2004">{{cite book |editor-last1=Weishampel |editor-first1=David B. |editor-last2=Dodson |editor-first2=Peter |editor-last3=Osmólska |editor-first3=Halszka |year=2004 |title=The Dinosauria |edition=Second |publisher=University of California Press |pages=861 pp}}</ref><ref name="senter2007">{{cite journal |last1=Senter |first1=P. |year=2007 |title=A new look at the phylogeny of Coelurosauria (Dinosauria: Theropoda) |journal=Journal of Systematic Palaeontology |volume=5| issue=4| pages=429–463| doi=10.1017/S1477201907002143 |bibcode=2007JSPal...5..429S }}</ref> with ''[[Troodon]]'' being sometimes added as a second external specifier in case it is closer to birds than to ''Deinonychus''.<ref>{{Cite journal |last1=Maryańska|first1=Teresa|last2=Osmólska|first2=Halszka|last3=Wolsan|first3=Mieczysław |date=2002|title=Avialan status for Oviraptorosauria|journal=Acta Palaeontologica Polonica}}</ref> Avialae is also occasionally defined as an [[Phylogenetic nomenclature#Phylogenetic definitions|apomorphy-based clade]] (that is, one based on physical characteristics). [[Jacques Gauthier]], who named Avialae in 1986, re-defined it in 2001 as all dinosaurs that possessed feathered wings used in flapping [[Bird flight|flight]], and the birds that descended from them.<ref name="gauthier&dequeiroz2001">{{cite book |last1=Gauthier |first1=J. |last2=de Queiroz |first2=K. |year=2001 |chapter=Feathered dinosaurs, flying dinosaurs, crown dinosaurs, and the name Aves |pages=7–41 |title=New perspectives on the origin and early evolution of birds: proceedings of the International Symposium in Honor of John H. Ostrom |editor-first1=J. A. |editor-last1=Gauthier |editor-first2=L. F. |editor-last2=Gall |publisher=Peabody Museum of Natural History, Yale University |location=New Haven, CT}}</ref><ref name="gauthier1986">{{cite book |last1=Gauthier |first1=J. |year=1986 |chapter=Saurischian monophyly and the origin of birds |editor-first1=K. |editor-last1=Padian |title=The origin of birds and the evolution of flight |location=San Francisco, California |publisher=Mem. Calif. Acad. Sci. |pages=1–55}}</ref>

Despite being currently one of the most widely used, the crown-group definition of Aves has been criticised by some researchers. Lee and Spencer (1997) argued that, contrary to what Gauthier defended, this definition would not increase the stability of the clade and the exact content of Aves will always be uncertain because any defined clade (either crown or not) will have few synapomorphies distinguishing it from its closest relatives. Their alternative definition is synonymous to Avifilopluma.<ref name=":1">{{cite book |doi=10.1016/B978-012676460-4/50004-4 |chapter=Crown-Clades, Key Characters and Taxonomic Stability: When is an Amniote Not an Amniote? |title=Amniote Origins |date=1997 |last1=Lee |first1=Michael S.Y. |last2=Spencer |first2=Patrick S. |pages=61–84 |isbn=978-0-12-676460-4 }}</ref>

===Dinosaurs and the origin of birds===
{{Main|Origin of birds}}
{{Cladogram|caption=Cladogram following the results of a phylogenetic study by Cau ''et al.'', 2015<ref name=cauetal2015>{{cite journal|doi=10.7717/peerj.1032|pmid=26157616|pmc=4476167|title=The phylogenetic affinities of the bizarre Late Cretaceous Romanian theropod ''Balaur'' bondoc(Dinosauria, Maniraptora): Dromaeosaurid or flightless bird?|journal=PeerJ|volume=3|pages=e1032|year=2015|last1=Cau|first1=Andrea|last2=Brougham|first2=Tom|last3=Naish |first3=Darren |doi-access=free }}</ref> |clades=
{{clade| style=font-size:75%;line-height:80%
|label1=[[Paraves]]
|1={{clade
  |1={{extinct}}[[Scansoriopterygidae]]
  |2={{clade
    |1={{extinct}}''[[Eosinopteryx]]''
    |label2=[[Eumaniraptora]]
    |2={{clade
     |1=†''[[Jinfengopteryx]]''
     |2=†''[[Aurornis]]''
     |3=†[[Dromaeosauridae]]
     |4=†[[Troodontidae]]
     |5=[[Avialae]] }} }} }} }} }}
[[File:Birds and dinosaurs.webp|left|thumb|upright=1.35|Simplified [[phylogenetic tree]] showing the relationship between modern birds and other dinosaurs<ref>{{Cite journal|last1=Plotnick|first1=Roy E.|last2=Theodor|first2=Jessica M. |last3=Holtz|first3=Thomas R.|date=24 September 2015|title=Jurassic Pork: What Could a Jewish Time Traveler Eat?|journal=Evolution: Education and Outreach|volume=8|issue=1|pages=17 |doi=10.1186/s12052-015-0047-2 |doi-access=free|hdl=1903/27622|hdl-access=free}}</ref>]]

Based on fossil and biological evidence, most scientists accept that birds are a specialised subgroup of [[theropod]] [[dinosaur]]s<ref>{{Cite journal|last=Prum|first=Richard O. |title=Who's Your Daddy?|journal=Science|volume=322|pages=1799–1800|date=19 December 2008|doi=10.1126/science.1168808|pmid=19095929|issue=5909 |doi-access=free}}</ref> and, more specifically, members of [[Maniraptora]], a group of theropods which includes [[Dromaeosauridae|dromaeosaurids]] and [[Oviraptorosauria|oviraptorosaurs]], among others.<ref>{{Cite book|last=Paul |first=Gregory S. |author-link=Gregory S. Paul |chapter=Looking for the True Bird Ancestor |year=2002 |title=Dinosaurs of the Air: The Evolution and Loss of Flight in Dinosaurs and Birds |url=https://archive.org/details/dinosaursofairev0000paul |location=Baltimore|publisher=Johns Hopkins University Press |isbn=0-8018-6763-0|pages=[https://archive.org/details/dinosaursofairev0000paul/page/171 171]–224}}</ref> As scientists have discovered more theropods closely related to birds, the previously clear distinction between non-birds and birds has become blurred. By the 2000s, discoveries in the [[Liaoning]] Province of northeast China, which demonstrated many small theropod [[feathered dinosaur]]s, contributed to this ambiguity.<ref>{{Cite book |last=Norell |first=Mark |author2=Mick Ellison |year=2005 |title=Unearthing the Dragon: The Great Feathered Dinosaur Discovery |location=New York |publisher=Pi Press |isbn=0-13-186266-9 |url=https://archive.org/details/unearthingdragon00mark }}</ref><ref name="AP-20140731">{{cite news |last=Borenstein |first=Seth |title=Study traces dinosaur evolution into early birds |url=http://apnews.excite.com/article/20140731/us-sci-shrinking-dinosaurs-a5c053f221.html |date=31 July 2014 |agency=Associated Press |access-date=3 August 2014 |archive-url=https://web.archive.org/web/20140808042331/http://apnews.excite.com/article/20140731/us-sci-shrinking-dinosaurs-a5c053f221.html |archive-date=8 August 2014 }}</ref><ref name="SCI-20140731">{{cite journal |title=Sustained miniaturization and anatomical innovation in the dinosaurian ancestors of birds |date=1 August 2014 |journal=[[Science (journal)|Science]] |volume=345 |issue=6196 |pages=562–566 |doi=10.1126/science.1252243 |last1=Lee |first1=Michael S. Y. |first2=Andrea|last2=Cau |first3=Darren|last3=Naish|first4=Gareth J.|last4=Dyke |bibcode=2014Sci...345..562L |pmid=25082702 }}</ref>

[[File:Anchiornis feathers.jpg|thumb|''[[Anchiornis huxleyi]]'' is an important source of information on the early evolution of birds in the [[Late Jurassic]] period.<ref name="lietal2010">{{cite journal |last1=Li |first1=Q. |last2=Gao |first2=K.-Q. |last3=Vinther |first3=J. |last4=Shawkey |first4=M. D. |last5=Clarke |first5=J. A. |last6=d'Alba |first6=L. |last7=Meng |first7=Q. |last8=Briggs |first8=D. E. G. |last9=Prum |first9=R. O. |year=2010 |name-list-style=amp |title=Plumage color patterns of an extinct dinosaur |journal=[[Science (journal)|Science]] |volume=327 |issue=5971 |pages=1369–1372 |bibcode=2010Sci...327.1369L |doi=10.1126/science.1186290 |pmid=20133521 |url=http://doc.rero.ch/record/210394/files/PAL_E4402.pdf }}</ref>]]
The consensus view in contemporary [[paleontology|palaeontology]] is that the flying theropods, or [[Avialae|avialans]], are the closest relatives of the [[deinonychosaur]]s, which include dromaeosaurids and [[troodontid]]s.<ref name=Xiaotingia>{{cite journal |title=An ''Archaeopteryx''-like theropod from China and the origin of Avialae |date=28 July 2011 |journal=Nature |volume=475 |pages=465–470 |doi=10.1038/nature10288 |issue=7357 |author1=Xing Xu |author2=Hailu You |author3=Kai Du |author4=Fenglu Han |pmid=21796204 }}</ref> Together, these form a group called [[Paraves]]. Some [[basal (phylogenetics)|basal]] members of Deinonychosauria, such as ''[[Microraptor]]'', have features which may have enabled them to glide or fly. The most basal deinonychosaurs were very small. This evidence raises the possibility that the ancestor of all paravians may have been [[arboreal]], have been able to glide, or both.<ref name="AHTetal07">{{Cite journal|last1=Turner |first1=Alan H. |last2=Pol |first2=D. |last3=Clarke |first3=J. A. |last4=Erickson |first4=G. M. |last5=Norell |first5=M. A. |date=7 September 2007 |title=A basal dromaeosaurid and size evolution preceding avian flight |journal=[[Science (journal)|Science]] |volume=317 |pages=1378–1381 |doi=10.1126/science.1144066 |pmid=17823350 |issue=5843 |bibcode=2007Sci...317.1378T |doi-access=free }}</ref><ref name="xuetal2003">{{Cite journal|date=23 January 2003|title=Four-winged dinosaurs from China|journal=[[Nature (journal)|Nature]]|volume=421|issue=6921|pages=335–340|doi=10.1038/nature01342|pmid=12540892 |last1=Xu |first1=X. |last2=Zhou |first2=Z. |last3=Wang |first3=X. |last4=Kuang |first4=X. |last5=Zhang |first5=F. |last6=Du |first6=X. |bibcode=2003Natur.421..335X |url=http://doc.rero.ch/record/15275/files/PAL_E2574.pdf }}</ref> Unlike ''Archaeopteryx'' and the non-avialan feathered dinosaurs, who primarily ate meat, studies suggest that the first avialans were [[omnivore]]s.<ref>{{cite web |url=http://the-scientist.com/2011/07/27/on-the-origin-of-birds |title=On the Origin of Birds |access-date=11 June 2012 |author=Luiggi, Christina |date=July 2011 |publisher=The Scientist |archive-url=https://web.archive.org/web/20120616171500/http://the-scientist.com/2011/07/27/on-the-origin-of-birds/ |archive-date=16 June 2012 }}</ref>

The [[Late Jurassic]] ''Archaeopteryx'' is well known as one of the first [[transitional fossil]]s to be found, and it provided support for the [[theory of evolution]] in the late 19th century. ''Archaeopteryx'' was the first fossil to display both clearly traditional reptilian characteristics—teeth, clawed fingers, and a long, lizard-like tail—as well as wings with flight feathers similar to those of modern birds. It is not considered a direct ancestor of birds, though it is possibly closely related to the true ancestor.<ref name="mayretal2007">{{Cite journal |doi=10.1111/j.1096-3642.2006.00245.x |last1=Mayr |first1=G. |last2=Pohl |first2=B. |last3=Hartman |first3=S. |last4=Peters |first4=D. S. |date=January 2007 |title=The tenth skeletal specimen of ''Archaeopteryx''| journal=Zoological Journal of the Linnean Society |volume=149 |issue=1 |pages=97–116 |doi-access=free}}</ref>

===Early evolution===
{{See also|List of fossil bird genera}}
[[File:Confuciusornis male.jpg|thumb|left|alt= White slab of rock left with cracks and impression of bird feathers and bone, including long paired tail feathers|''[[Confuciusornis sanctus]]'', a Cretaceous bird from China that lived 125&nbsp;million years ago, is the oldest known bird to have a beak.<ref>{{cite book |last1=Ivanov |first1=M. |last2=Hrdlickova |first2=S. |last3=Gregorova |first3=R. |year=2001 |title=The Complete Encyclopedia of Fossils |publisher=Rebo Publishers |location=Netherlands |page=312}}</ref>]]

Over 40% of key traits found in modern birds evolved during the 60&nbsp;million year transition from the earliest [[Avemetatarsalia|bird-line archosaurs]] to the first [[Maniraptoromorpha|maniraptoromorphs]], i.e. the first dinosaurs closer to living birds than to ''[[Tyrannosaurus|Tyrannosaurus rex]]''. The loss of osteoderms otherwise common in archosaurs and acquisition of primitive feathers might have occurred early during this phase.<ref name="paleoitalia">{{Cite journal |last=Cau |first=Andrea |date=2018 |title=The assembly of the avian body plan: a 160-million-year long process |url=http://paleoitalia.org/media/u/archives/01_Cau_2018_BSPI_571.pdf |url-status=live |journal=Bollettino della Società Paleontologica Italiana |archive-url=https://web.archive.org/web/20180505065903/http://paleoitalia.org/media/u/archives/01_Cau_2018_BSPI_571.pdf |archive-date=5 May 2018}}</ref><ref name="Benton Dhouailly Early Origin">{{cite journal |last1=Benton |first1=Michael J. |last2=Dhouailly |first2=Danielle |last3=Jiang |first3=Baoyu |last4=McNamara |first4=Maria |title=The Early Origin of Feathers |journal=Trends in Ecology & Evolution |date=September 2019 |volume=34 |issue=9 |pages=856–869 |doi=10.1016/j.tree.2019.04.018 |pmid=31164250 |bibcode=2019TEcoE..34..856B |hdl=10468/8068 |hdl-access=free }}</ref> After the appearance of Maniraptoromorpha, the next 40&nbsp;million years marked a continuous reduction of body size and the accumulation of [[Neoteny|neotenic]] (juvenile-like) characteristics. [[Hypercarnivore|Hypercarnivory]] became increasingly less common while braincases enlarged and forelimbs became longer.<ref name="paleoitalia"/> The [[integument]] evolved into complex, [[pennaceous feather]]s.<ref name="Benton Dhouailly Early Origin"/>

The oldest known paravian (and probably the earliest avialan) fossils come from the [[Tiaojishan Formation]] of China, which has been dated to the late [[Jurassic]] period ([[Oxfordian (stage)|Oxfordian]] stage), about 160&nbsp;million years ago. The avialan species from this time period include ''[[Anchiornis huxleyi]]'', ''[[Xiaotingia zhengi]]'', and ''[[Aurornis xui]]''.<ref name="Nature" />

The well-known probable early avialan, ''Archaeopteryx'', dates from slightly later Jurassic rocks (about 155&nbsp;million years old) from [[Germany]]. Many of these early avialans shared unusual anatomical features that may be ancestral to modern birds but were later lost during bird evolution. These features include enlarged claws on the second toe which may have been held clear of the ground in life, and long feathers or "hind wings" covering the hind limbs and feet, which may have been used in aerial maneuvering.<ref name="zhengetal2013">{{cite journal | last1 = Zheng | first1 = X. | last2 = Zhou | first2 = Z. | last3 = Wang | first3 = X. | last4 = Zhang | first4 = F. | last5 = Zhang | first5 = X. | last6 = Wang | first6 = Y. | last7 = Wei | first7 = G. | last8 = Wang | first8 = S. | last9 = Xu | first9 = X. | date = 15 March 2013 | title = Hind Wings in Basal Birds and the Evolution of Leg Feathers | journal = Science | volume = 339 | issue = 6125| pages = 1309–1312 | doi = 10.1126/science.1228753 | pmid=23493711 | bibcode = 2013Sci...339.1309Z }}</ref>

Avialans diversified into a wide variety of forms during the [[Cretaceous]] period. Many groups retained [[symplesiomorphy|primitive characteristics]], such as clawed wings and teeth, though the latter were lost independently in a number of avialan groups, including modern birds (Aves).<ref name="chiappe2007">{{Cite book|last=Chiappe |first=Luis M. |year=2007 |title=Glorified Dinosaurs: The Origin and Early Evolution of Birds |location=Sydney |publisher=University of New South Wales Press |isbn=978-0-86840-413-4}}</ref> Increasingly stiff tails (especially the outermost half) can be seen in the evolution of maniraptoromorphs, and this process culminated in the appearance of the [[pygostyle]], an ossification of fused tail vertebrae.<ref name="paleoitalia"/> In the late Cretaceous, about 100&nbsp;million years ago, the ancestors of all modern birds evolved a more open pelvis, allowing them to lay larger eggs compared to body size.<ref>{{cite journal |last1=Pickrell |first1=John |title=Early birds may have been too hefty to sit on their eggs |journal=Nature |date=22 March 2018 |doi=10.1038/d41586-018-03447-3 }}</ref> Around 95&nbsp;million years ago, they evolved a better sense of smell.<ref>{{cite web | url=http://archive.cosmosmagazine.com/news/birds-survived-dino-extinction-with-keen-senses/ | title=Birds survived dino extinction with keen senses | access-date=11 June 2012 | author=Agency France-Presse | date=April 2011 | publisher=Cosmos Magazine | archive-url=https://web.archive.org/web/20150402124421/http://archive.cosmosmagazine.com/news/birds-survived-dino-extinction-with-keen-senses/ | archive-date=2 April 2015}}</ref>

A third stage of bird evolution starting with [[Ornithothoraces]] (the "bird-chested" avialans) can be associated with the refining of aerodynamics and flight capabilities, and the loss or co-ossification of several skeletal features. Particularly significant are the development of an enlarged, [[Keel (bird anatomy)|keeled]] sternum and the [[alula]], and the loss of grasping hands.
<ref name="paleoitalia"/> {{Cladogram|caption=Cladogram following the results of a phylogenetic study by Cau ''et al.'', 2015<ref name=cauetal2015/>|clades=
{{clade| style=font-size:75%;line-height:80%
     |label1=[[Avialae]]
     |1={{clade
        |1=†''[[Anchiornis]]''
        |2={{clade
        |1=†''[[Archaeopteryx]]''
        |2={{clade
        |1=†''[[Xiaotingia]]''
        |2={{clade
           |1=†''[[Rahonavis]]''
            |2={{clade
              |1={{clade
              |1=†''[[Jeholornis]]''
              |2=†''[[Jixiangornis]]'' }}
           |label2=[[Euavialae]]
           |2={{clade
              |1=†''[[Balaur bondoc|Balaur]]''
              |label2=[[Avebrevicauda]]
              |2={{clade
              |1=†''[[Zhongjianornis]]''
              |2={{clade
                 |1=†''[[Sapeornis]]''
                 |label2=[[Pygostylia]]
                 |2={{clade
                     |1={{extinct}}[[Confuciusornithiformes]]
                     |2={{clade
                        |1={{clade
                           |1=†''[[Protopteryx]]''
                           |2=†''[[Pengornis]]'' }}
                     |2=[[Ornithothoraces]] }} }} }} }} }} }} }} }} }} }} }} }}

===Early diversity of bird ancestors===
{{See also| Protobirds|Avialae}}
{{Cladogram|caption=Mesozoic bird phylogeny simplified after Wang et al., 2015's phylogenetic analysis<ref name=Wang2015>{{Cite journal
| doi = 10.1038/ncomms7987
| pmid = 25942493
| title = The oldest record of ornithuromorpha from the early cretaceous of China
| journal = Nature Communications
| volume = 6
| number = 6987
| year = 2015
| last1 = Wang | first1 = M.
| last2 = Zheng | first2 = X.
| last3 = O'Connor | first3 = J. K.
| last4 = Lloyd | first4 = G. T.
| last5 = Wang | first5 = X.
| last6 = Wang | first6 = Y.
| last7 = Zhang | first7 = X.
| last8 = Zhou | first8 = Z. | page=6987
| bibcode = 2015NatCo...6.6987W| pmc = 5426517
}}</ref>

|clades= {{clade| style=font-size:75%;line-height:80%
  |label1=[[Ornithothoraces]]
  |1={{clade
    |1=†[[Enantiornithes]]
    |label2=[[Euornithes]]
    |2={{clade
   |1=†''[[Archaeorhynchus]]''
   |label2=[[Ornithuromorpha]]
   |2={{Clade
      |1=''†[[Patagopteryx]]''
      |2=''†[[Vorona]]''
      |label3=
      |3={{Clade
         |1=''†[[Schizooura]]''
         |label2=
         |2={{Clade
            |1=†[[Hongshanornithidae]]
            |label2=
            |2={{Clade
               |1=''†[[Jianchangornis]]''
               |label2=
               |2={{Clade
                  |1=†[[Songlingornithidae]]
                  |label2=
                  |2={{Clade
                     |1=''†[[Gansus]]''
                     |label2=
                     |2={{Clade
                        |1=''†[[Apsaravis]]''
                        |label2=[[Ornithurae]]
                        |2={{clade
                          |1={{extinct}}[[Hesperornithes]]
                          |2={{clade
                             |1=†''[[Ichthyornis]]''
                             |2={{clade
                                |1=†''[[Vegavis]]''
                                |2='''Aves'''
                     }}
                  }}
               }}
            }}
         }}
      }}
   }}
}} }} }} }} }} }} }}
[[File:Ichthyornis Clean.png|thumb|upright|''[[Ichthyornis]]'', which lived 93&nbsp;million years ago, was the first known prehistoric bird relative preserved with teeth.]]
The first large, diverse lineage of short-tailed avialans to evolve were the [[Enantiornithes]], or "opposite birds", so named because the construction of their shoulder bones was in reverse to that of modern birds. Enantiornithes occupied a wide array of [[ecological niche]]s, from sand-probing shorebirds and fish-eaters to tree-dwelling forms and seed-eaters. While they were the dominant group of avialans during the Cretaceous period, Enantiornithes became extinct along with many other dinosaur groups at the end of the [[Mesozoic]] era.<ref name="chiappe2007"/><ref>{{Cite web |last=Elbein |first=Asher |title=Why Do Birds Have Such Skinny Legs? |url=https://www.scientificamerican.com/article/why-do-birds-have-such-skinny-legs/ |access-date=15 February 2024 |website=Scientific American |language=en}}</ref>

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 euornithians lacked [[Perching bird|perching]] adaptations and likely included shorebird-like species, waders, and swimming and diving species.<ref name="brusatte2015">{{cite journal |last1=Brusatte |first1=S.L. |last2=O'Connor |first2=J.K. |last3=Jarvis |first3=J.D. |year=2015 |title=The Origin and Diversification of Birds |journal=Current Biology |volume=25 |issue=19 |pages=R888–R898 |doi=10.1016/j.cub.2015.08.003|pmid=26439352 |doi-access=free |bibcode=2015CBio...25.R888B |hdl=10161/11144 |hdl-access=free }}</ref>

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 |hdl=2246/454 |url=http://publication.plazi.org/id/BF3BCD16FFE2FF9AFFB377697C7DB221 }}</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 euornithians 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 euornithians retained teeth in other parts of the jaws).<ref name=louchart2011>{{cite journal |last1=Louchart |first1=Antoine |last2=Viriot |first2=Laurent |title=From snout to beak: the loss of teeth in birds |journal=Trends in Ecology & Evolution |date=December 2011 |volume=26 |issue=12 |pages=663–673 |doi=10.1016/j.tree.2011.09.004 |pmid=21978465 |bibcode=2011TEcoE..26..663L }}</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 | bibcode = 2018PNAS..115..555F | doi-access = free }}</ref>

===Diversification of modern birds===
{{See also|Sibley–Ahlquist taxonomy of birds|dinosaur classification}}
{{Cladogram|caption=Major groups of modern birds based on [[Sibley-Ahlquist taxonomy]]
|clades={{clade | style=font-size:85%;line-height:100%;width:325px;
|label1='''Aves'''
|1={{clade
 |label1=[[Palaeognathae]]
 |1=([[ratites]] and [[tinamous]])
 |label2= [[Neognathae]]
 |2={{clade
    |2=(all other birds including [[passerine|perching]] birds)
    |label2=[[Neoaves]]
    |label1=[[Galloanserae]]
    |1=([[landfowl]] and [[waterfowl]])
    }}
   }}
}}
}}

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">{{cite journal |author=Yonezawa, T. |display-authors=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 |bibcode=2017CBio...27...68Y }}</ref><ref name=divergence>{{cite journal |last1=Lee |first1=M. S. Y. |last2=Cau |first2=A. |last3=Naish |first3=D. |last4=Dyke |first4=G. J. |title=Morphological Clocks in Paleontology, and a Mid-Cretaceous Origin of Crown Aves |journal=Systematic Biology |date=May 2014 |volume=63 |issue=3 |pages=442–449 |doi=10.1093/sysbio/syt110 |pmid=24449041 |doi-access=free }}</ref> to the latest 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 }}</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 |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> Similarly, there is no agreement on whether most of the early diversification of modern birds occurred in the Cretaceous and associated with breakup of the supercontinent [[Gondwana]] or occurred later and potentially as a consequence of 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=C. L. |last3=Britton |first3=T. |last4=Elzanowski |first4=A. |last5=Johansson |first5=U. S. |last6=Källersjö |first6=M. |last7=Ohlson |first7=J. I. |last8=Parsons |first8=T. J. |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 }}</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).

The discovery in 2005 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 |hdl=11336/80763 |hdl-access=free }}</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) | 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 | url = http://publication.plazi.org/id/BF3BCD16FFE2FF9AFFB377697C7DB221 }}</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"/>

Attempts to reconcile molecular and fossil evidence using genomic-scale DNA data and comprehensive fossil information have not resolved the controversy.<ref name=Prum2015/><ref name=Jarvis2014/> However, a 2015 estimate that used a new method for calibrating [[molecular clocks]] confirmed that while modern birds originated early in the Late Cretaceous, likely in Western [[Gondwana]], a pulse of diversification in all major groups occurred around the Cretaceous–Palaeogene extinction event.<ref name=Claramunt2015>{{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> Modern birds would have expanded from West Gondwana through two routes. One route was an Antarctic interchange in the Paleogene. The other route was probably via Paleocene land bridges between South America and North America, which allowed for the rapid expansion and diversification of Neornithes into the [[Holarctic]] and [[Paleotropics]].<ref name=Claramunt2015/> On the other hand, the occurrence of ''Asteriornis'' in the Northern Hemisphere suggest that Neornithes dispersed out of East Gondwana before the Paleocene.<ref name="Field2020"/>

===Classification of bird orders===
{{See also|List of birds}}

All modern birds lie within the [[crown group]] Aves (alternately Neornithes), which has two subdivisions: the [[Palaeognathae]], which includes the flightless [[ratite]]s (such as the [[ostrich]]es) and the weak-flying [[tinamou]]s, and the extremely diverse [[Neognathae]], containing all other birds.<ref name="Mitchell2014">{{Cite journal | doi = 10.1126/science.1251981| pmid = 24855267| title = Ancient DNA reveals elephant birds and kiwi are sister taxa and clarifies ratite bird evolution| journal = Science| volume = 344| issue = 6186| pages = 898–900| date = 23 May 2014| last1 = Mitchell | first1 = K. J.| last2 = Llamas | first2 = B.| last3 = Soubrier | first3 = J.| last4 = Rawlence | first4 = N. J.| last5 = Worthy | first5 = T. H.| last6 = Wood | first6 = J.| last7 = Lee | first7 = M. S. Y.| last8 = Cooper | first8 = A.| bibcode = 2014Sci...344..898M| hdl = 2328/35953 | hdl-access = free}}</ref> These two subdivisions have variously been given the [[taxonomic rank|rank]] of [[superorder]],<ref>{{cite web|url=http://people.eku.edu/ritchisong/birdbiogeography1.htm |title=Bird biogeography | last= Ritchison| first=Gary |access-date=10 April 2008 |work=Avian Biology|publisher=Eastern Kentucky University}}</ref> cohort,<ref name="Theropoda"/> or infraclass.<ref>{{cite book |last1=Cracraft |first1=J. |chapter=Avian Higher-level Relationships and Classification: Nonpasseriforms |pages=xxi–xli |editor-last1=Dickinson |editor-first1=E. C.|editor-first2=J. V. |editor-last2=Remsen|year=2013 |title=The Howard and Moore Complete Checklist of the Birds of the World |edition=4th |volume=1 |publisher=Aves Press, Eastbourne, U.K.}}</ref> The number of known living bird species is around 11,000<ref name="IOC14.2">{{cite taxon|IOC|version=14.2|doi=|doi-access=|access-date=21 November 2024}}</ref><ref>{{Cite web |title=October 2022 {{!}} Clements Checklist |url=https://www.birds.cornell.edu/clementschecklist/updateindex/october-2022/ |access-date=6 January 2023 |website=www.birds.cornell.edu}}</ref> although sources may differ in their precise numbers.

[[Cladogram]] of modern bird relationships based on Stiller ''et al'' (2024).,<ref>{{cite journal |last1=Stiller |first1=Josefin |last2=Feng |first2=Shaohong |last3=Chowdhury |first3=Al-Aabid |last4=Rivas-González |first4=Iker |last5=Duchêne |first5=David A. |last6=Fang |first6=Qi |last7=Deng |first7=Yuan |last8=Kozlov |first8=Alexey |last9=Stamatakis |first9=Alexandros |last10=Claramunt |first10=Santiago |last11=Nguyen |first11=Jacqueline M. T. |last12=Ho |first12=Simon Y. W. |last13=Faircloth |first13=Brant C. |last14=Haag |first14=Julia |last15=Houde |first15=Peter |last16=Cracraft |first16=Joel |last17=Balaban |first17=Metin |last18=Mai |first18=Uyen |last19=Chen |first19=Guangji |last20=Gao |first20=Rongsheng |last21=Zhou |first21=Chengran |last22=Xie |first22=Yulong |last23=Huang |first23=Zijian |last24=Cao |first24=Zhen |last25=Yan |first25=Zhi |last26=Ogilvie |first26=Huw A. |last27=Nakhleh |first27=Luay |last28=Lindow |first28=Bent |last29=Morel |first29=Benoit |last30=Fjeldså |first30=Jon |last31=Hosner |first31=Peter A. |last32=da Fonseca |first32=Rute R. |last33=Petersen |first33=Bent |last34=Tobias |first34=Joseph A. |last35=Székely |first35=Tamás |last36=Kennedy |first36=Jonathan David |last37=Reeve |first37=Andrew Hart |last38=Liker |first38=Andras |last39=Stervander |first39=Martin |last40=Antunes |first40=Agostinho |last41=Tietze |first41=Dieter Thomas |last42=Bertelsen |first42=Mads F. |last43=Lei |first43=Fumin |last44=Rahbek |first44=Carsten |last45=Graves |first45=Gary R. |last46=Schierup |first46=Mikkel H. |last47=Warnow |first47=Tandy |last48=Braun |first48=Edward L. |last49=Gilbert |first49=M. Thomas P. |last50=Jarvis |first50=Erich D. |last51=Mirarab |first51=Siavash |last52=Zhang |first52=Guojie |title=Complexity of avian evolution revealed by family-level genomes |journal=Nature |date=23 May 2024 |volume=629 |issue=8013 |pages=851–860 |doi=10.1038/s41586-024-07323-1 |pmid=38560995 |pmc=11111414 |bibcode=2024Natur.629..851S }}</ref> showing the 44 orders recognised by the IOC.<ref name=IOC14.2/>

{{clade transclude
|page=Template:Phylogeny/Birds
|label=Aves
|wrap='''Aves'''
|nohidden=yes}}

The classification of birds is a contentious issue. [[Charles Sibley|Sibley]] and [[Jon Ahlquist|Ahlquist]]'s ''Phylogeny and Classification of Birds'' (1990) is a landmark work on the subject.<ref>{{Cite book|last=Sibley |first=Charles |author-link=Charles Sibley |author2=Jon Edward Ahlquist |year=1990 |title=Phylogeny and classification of birds |location=New Haven |publisher=Yale University Press |isbn=0-300-04085-7|author2-link=Jon Edward Ahlquist }}</ref> Most evidence seems to suggest the assignment of orders is accurate,<ref>{{Cite book|last=Mayr |first=Ernst |author-link=Ernst W. Mayr |author2=Short, Lester L.|title=Species Taxa of North American Birds: A Contribution to Comparative Systematics |series=Publications of the Nuttall Ornithological Club, no. 9 |year=1970 |publisher=Nuttall Ornithological Club|location=Cambridge, MA |oclc=517185}}</ref> but scientists disagree about the relationships among the orders themselves; evidence from modern bird anatomy, fossils and DNA have all been brought to bear on the problem, but no strong consensus has emerged. Fossil and molecular evidence from the 2010s is providing an increasingly clear picture of the evolution of modern bird orders.<ref name=Prum2015/><ref name=Jarvis2014>{{cite journal | last1 = Jarvis | first1 = E. D. | display-authors = et al | year = 2014 | title = Whole-genome analyses resolve early branches in the tree of life of modern birds | journal = Science | volume = 346 | issue = 6215| pages = 1320–1331 | doi=10.1126/science.1253451 | pmid=25504713 | pmc=4405904| bibcode = 2014Sci...346.1320J }}</ref>

===Genomics===
{{See also|list of sequenced animal genomes#Birds}}
In 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 |doi-access=free |url=https://knowablemagazine.org/article/living-world/2022/learning-about-birds-their-genomes |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 }}</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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Being able to sequence and compare whole genomes gives researchers many types of information, about genes, the DNA that regulates the genes, and their evolutionary history. This has led to reconsideration of some of the classifications that were based solely on the identification of protein-coding genes. Waterbirds such as [[pelicans]] and [[flamingos]], for example, may have in common specific adaptations suited to their environment that were developed independently.<ref name="Holmes"/><ref name="Bravo"/>