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Bird
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Birds
Fossilrange:150–0MaPreЄЄOSDCPTJKPgNLateJurassic–Recent

Double-crestedCormorant,Phalacrocoraxauritus
Scientificclassification
Kingdom:Animalia

Phylum:Chordata

Subphylum:Vertebrata

(unranked):Archosauria

Class:Aves
Linnaeus,1758

Orders
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MorespokenarticlesBirds(classAves)arebipedal,endothermic(warm-blooded),vertebrateanimalsthatlayeggs.Therearearound10,000livingspecies,makingthemthemostdiversetetrapodvertebrates.Theyinhabitecosystemsacrosstheglobe,fromtheArctictotheAntarctic.Birdsrangeinsizefromthe5cm(2in)BeeHummingbirdtothe2.7m(9ft)Ostrich.ThefossilrecordindicatesthatbirdsevolvedfromtheropoddinosaursduringtheJurassicperiod,around150–200Ma(millionyearsago),andtheearliestknownbirdistheLateJurassicarchaeopteryx,c155–150Ma.MostpaleontologistsregardbirdsastheonlycladeofdinosaursthatsurvivedtheCretaceous–Tertiaryextinctioneventapproximately65.5Ma.

ModernbirdsarecharacterisedbyFEATHERS,abeakwithnoteeth,thelayingofhard-shelledeggs,ahighmetabolicrate,afour-chamberedheart,andalightweightbutstrongskeleton.Allbirdshaveforelimbsmodifiedaswingsandmostcanfly,withsomeexceptionsincludingratites,penguins,andanumberofdiverseendemicislandspecies.Birdsalsohaveuniquedigestiveandrespiratorysystemsthatarehighlyadaptedforflight.Somebirds,especiallycorvidsandparrots,areamongthemostintelligentanimalspecies;anumberofbirdspecieshavebeenobservedmanufacturingandusingtools,andmanysocialspeciesexhibitculturaltransmissionofknowledgeacrossgenerations.

Manyspeciesundertakelongdistanceannualmigrations,andmanymoreperformshorterirregularmovements.Birdsaresocial;theycommunicateusingvisualsignalsandthroughcallsandsongs,andparticipateinsocialbehavioursincludingcooperativebreedingandhunting,flocking,andmobbingofpredators.Thevastmajorityofbirdspeciesaresociallymonogamous,usuallyforonebreedingseasonatatime,sometimesforyears,butrarelyforlife.Otherspecieshavebreedingsystemsthatarepolygynous("manyfemales")or,rarely,polyandrous("manymales").Eggsareusuallylaidinanestandincubatedbytheparents.Mostbirdshaveanextendedperiodofparentalcareafterhatching.

Manyspeciesareofeconomicimportance,mostlyassourcesoffoodacquiredthroughhuntingorfarming.Somespecies,particularlysongbirdsandparrots,arepopularaspets.Otherusesincludetheharvestingofguano(droppings)foruseasafertiliser.Birdsfigureprominentlyinallaspectsofhumanculturefromreligiontopoetrytopopularmusic.About120–130specieshavebecomeextinctasaresultofhumanactivitysincethe17thcentury,andhundredsmorebeforethen.Currentlyabout1,200speciesofbirdsarethreatenedwithextinctionbyhumanactivities,thougheffortsareunderwaytoprotectthem.

Contents[hide]
1Evolutionandtaxonomy
1.1Dinosaursandtheoriginofbirds
1.1.1Alternativetheoriesandcontroversies
1.2Earlyevolutionofbirds
1.3Radiationofmodernbirds
1.3.1Modernbirdorders
2Distribution
3Anatomyandphysiology
3.1Chromosomes
3.2Feathers,plumage,andscales
3.3Flight
4Behaviour
4.1Dietandfeeding
4.2Migration
4.3Communication
4.4Flockingandotherassociations
4.5Restingandroosting
4.6Breeding
4.6.1Socialsystems
4.6.2Territories,nestingandincubation
4.6.3Parentalcareandfledging
4.6.4Broodparasites
5Ecology
6Relationshipwithhumans
6.1Economicimportance
6.2Religion,folkloreandculture
6.3Conservation
7References
8Externallinks

Evolutionandtaxonomy
Mainarticle:Birdevolution

Archaeopteryx,theearliestknownbirdThefirstclassificationofbirdswasdevelopedbyFrancisWillughbyandJohnRayintheir1676volumeOrnithologiae.[1]CarolusLinnaeusmodifiedthatworkin1758todevisethetaxonomicclassificationsystemcurrentlyinuse.[2]BirdsarecategorisedasthebiologicalclassAvesinLinnaeantaxonomy.PhylogenetictaxonomyplacesAvesinthedinosaurcladeTheropoda.[3]Avesandasistergroup,thecladeCrocodilia,togetherarethesolelivingmembersofthereptilecladeArchosauria.Phylogenetically,AvesiscommonlydefinedasalldescendantsofthemostrecentcommonancestorofmodernbirdsandArchaeopteryxlithographica.[4]Archaeopteryx,fromtheTithonianstageoftheLateJurassic(some150–145millionyearsago),istheearliestknownbirdunderthisdefinition.Others,includingJacquesGauthierandadherentsofthePhylocodesystem,havedefinedAvestoincludeonlythemodernbirdgroups,excludingmostgroupsknownonlyfromfossils,andassigningthem,instead,totheAvialae[5]inparttoavoidtheuncertaintiesabouttheplacementofArchaeopteryxinrelationtoanimalstraditionallythoughtofastheropoddinosaurs.

AllmodernbirdsliewithinthesubclassNeornithes,whichhastwosubdivisions:thePaleognathae,containingmostlyflightlessbirdslikeostriches,andthewildlydiverseNeognathae,containingallotherbirds.[3]Thesetwosubdivisionsareoftengiventherankofsuperorder,[6]althoughLivezey&Zusiassignedthem"cohort"rank.[3]Dependingonthetaxonomicviewpoint,thenumberofknownlivingbirdspeciesvariesanywherefrom9,800[7]to10,050.[8]

Dinosaursandtheoriginofbirds
Mainarticle:Originofbirds

Confuciusornis,aCretaceousbirdfromChinaFossilevidenceandintensivebiologicalanalyseshavedemonstratedbeyondanyreasonabledoubtthatbirdsaretheropoddinosaurs.Morespecifically,theyaremembersofManiraptora,agroupoftheropodswhichincludesdromaeosaursandoviraptorids,amongothers.[9]Asscientistsdiscovermorenon-aviantheropodsthatarecloselyrelatedtobirds,thepreviouslycleardistinctionbetweennon-birdsandbirdshasbecomeblurred.RecentdiscoveriesintheLiaoningProvinceofnortheastChina,whichdemonstratethatmanysmalltheropoddinosaurshadfeathers,contributetothisambiguity.[10]

Theconsensusviewincontemporarypaleontologyisthatthebirds,Aves,aretheclosestrelativesofthedeinonychosaurs,whichincludedromaeosauridsandtroodontids.Together,thesethreeformagroupcalledParaves.ThebasaldromaeosaurMicroraptorhasfeatureswhichmayhaveenabledittoglideorfly.Themostbasaldeinonychosaursareverysmall.Thisevidenceraisesthepossibilitythattheancestorofallparaviansmayhavebeenarboreal,and/ormayhavebeenabletoglide.[11][12]

TheLateJurassicArchaeopteryxiswell-knownasoneofthefirsttransitionalfossilstobefoundanditprovidedsupportforthetheoryofevolutioninthelate19thcentury.Archaeopteryxhasclearlyreptiliancharacters:teeth,clawedfingers,andalong,lizard-liketail,butithasfinelypreservedwingswithflightfeathersidenticaltothoseofmodernbirds.Itisnotconsideredadirectancestorofmodernbirds,butistheoldestandmostprimitivememberofAvesorAvialae,anditisprobablycloselyrelatedtotherealancestor.IthasevenbeensuggestedthatArchaeopteryxwasadinosaurthatwasnomorecloselyrelatedtobirdsthanwereotherdinosaurgroups,[13]andthatAvimimuswasmorelikelytobetheancestorofallbirdsthanArchaeopteryx.[14]

Alternativetheoriesandcontroversies
Therehavebeenmanycontroversiesinthestudyoftheoriginofbirds.Earlydisagreementsincludedwhetherbirdsevolvedfromdinosaursormoreprimitivearchosaurs.Withinthedinosaurcampthereweredisagreementsastowhetherornithischianortheropoddinosaurswerethemorelikelyancestors.[15]Althoughornithischian(bird-hipped)dinosaurssharethehipstructureofmodernbirds,birdsarethoughttohaveoriginatedfromthesaurischian(lizard-hipped)dinosaurs,andthereforeevolvedtheirhipstructureindependently.[16]Infact,abird-likehipstructureevolvedathirdtimeamongapeculiargroupoftheropodsknownastheTherizinosauridae.

ScientistsLarryMartinandAlanFeducciabelievethatbirdsarenotdinosaurs,butthatbirdsevolvedfromearlyarchosaurslikeLongisquama.Themajorityoftheirpublicationsarguedthatthesimilaritiesbetweenbirdsandmaniraptorandinosaurswereconvergent,andthatthetwowereunrelated.Inthelate1990stheevidencethatbirdsweremaniraptoransbecamealmostindisputable,soMartinandFeducciaadoptedamodifiedversionofahypothesisbydinosaurartistGregoryS.Paul;wheremaniraptoransaresecondarilyflightlessbirdsbut,[17]intheirversion,birdsevolveddirectlyfromLongisquama.Thusbirdsarestillnotdinosaurs,butneitheraremostoftheknownspeciesthatarecurrentlyclassifiedastheropoddinosaurs.Maniraptoransare,instead,flightless,archosaurian,birds.[18]Thistheoryiscontestedbymostpaleontologists.[19]Thefeaturescitedasevidenceofflightlessnessareinterpretedbymainstreampaleontologistsasexaptations,or"pre-adaptations",thatmaniraptoransinheritedfromtheircommonancestorwithbirds.

Protoavistexensiswasdescribedin1991asabirdolderthanArchaeopteryx.Criticshaveindicatedthatthefossilispoorlypreserved,extensivelyreconstructed,andmaybeachimera(madeupoffossilizedbonesfromseveraldifferentkindsofanimals).Thebraincaseismostlikelythatofaveryearlycoelurosaur[20]

Earlyevolutionofbirds
Seealso:Listoffossilbirds

Aves
Archaeopteryx

Pygostylia
Confuciusornithidae

Ornithothoraces
Enantiornithes

Ornithurae
Hesperornithiformes

Neornithes

BasalbirdphylogenysimplifiedafterChiappe,2007[21]
BirdsdiversifiedintoawidevarietyofformsduringtheCretaceousPeriod.[21]Manygroupsretainedprimitivecharacteristics,suchasclawedwingsandteeth,thoughthelatterwerelostindependentlyinanumberofbirdgroups,includingmodernbirds(Neornithes).Whiletheearliestforms,suchasArchaeopteryxandJeholornis,retainedthelongbonytailsoftheirancestors,[21]thetailsofmoreadvancedbirdswereshortenedwiththeadventofthepygostyleboneinthecladePygostylia.

Thefirstlarge,diverselineageofshort-tailedbirdstoevolveweretheEnantiornithes,or"oppositebirds",sonamedbecausetheconstructionoftheirshoulderboneswasinreversetothatofmodernbirds.Enantiornithesoccupiedawidearrayofecologicalniches,fromsand-probingshorebirdsandfish-eaterstotree-dwellingformsandseed-eaters.[21]Moreadvancedlineagesalsospecialisedineatingfish,likethesuperficiallygull-likesubclassofIchthyornithes("fishbirds").[22]OneorderofMesozoicseabirds,theHesperornithiformes,becamesowelladaptedtohuntingfishinmarineenvironmentsthattheylosttheabilitytoflyandbecameprimarilyaquatic.Despitetheirextremespecialisations,theHesperornithiformesrepresentsomeoftheclosestrelativesofmodernbirds.[21]

Radiationofmodernbirds
Seealso:Sibley-Ahlquisttaxonomyanddinosaurclassification
Containingallmodernbirds,thesubclassNeornithesis,duetothediscoveryofVegavis,nowknowntohaveevolvedintosomebasiclineagesbytheendoftheCretaceous[23]andissplitintotwosuperorders,thePaleognathaeandNeognathae.ThepaleognathsincludethetinamousofCentralandSouthAmericaandtheratites.ThebasaldivergencefromtheremainingNeognatheswasthatoftheGalloanserae,thesuperordercontainingtheAnseriformes(ducks,geese,swansandscreamers)andtheGalliformes(thepheasants,grouse,andtheirallies,togetherwiththemoundbuildersandtheguansandtheirallies).Thedatesforthesplitsaremuchdebatedbyscientists.ItisagreedthattheNeornithesevolvedintheCretaceous,andthatthesplitbetweentheGalloanserifromotherNeognathesoccurredbeforetheK–Textinctionevent,buttherearedifferentopinionsaboutwhethertheradiationoftheremainingNeognathesoccurredbeforeoraftertheextinctionoftheotherdinosaurs.[24]Thisdisagreementisinpartcausedbyadivergenceintheevidence;moleculardatingsuggestsaCretaceousradiation,whilefossilevidencesupportsaTertiaryradiation.Attemptstoreconcilethemolecularandfossilevidencehaveprovedcontroversial.[24][25]

Theclassificationofbirdsisacontentiousissue.SibleyandAhlquist'sPhylogenyandClassificationofBirds(1990)isalandmarkworkontheclassificationofbirds,[26]althoughitisfrequentlydebatedandconstantlyrevised.Mostevidenceseemstosuggestthattheassignmentofordersisaccurate,[27]butscientistsdisagreeabouttherelationshipsbetweentheordersthemselves;evidencefrommodernbirdanatomy,fossilsandDNAhaveallbeenbroughttobearontheproblem,butnostrongconsensushasemerged.Morerecently,newfossilandmolecularevidenceisprovidinganincreasinglyclearpictureoftheevolutionofmodernbirdorders.

Modernbirdorders

NeornithesPaleognathae
Struthioniformes

Tinamiformes

NeognathaeOtherbirds

Galloanserae
Anseriformes

Galliformes

Basaldivergencesofmodernbirds
basedonSibley-Ahlquisttaxonomy
ThisisalistofthetaxonomicordersinthesubclassNeornithes,ormodernbirds.Thislistusesthetraditionalclassification(theso-calledClementsorder),revisedbytheSibley-Monroeclassification.Thelistofbirdsgivesamoredetailedsummaryoftheorders,includingfamilies.

SubclassNeornithes
Paleognathae:

Struthioniformes—ostriches,emus,kiwis,andallies
Tinamiformes—tinamous
Neognathae:

Anseriformes—waterfowl
Galliformes—fowl
Charadriiformes—gulls,button-quails,ploversandallies
Gaviiformes—loons
Podicipediformes—grebes
Procellariiformes—albatrosses,petrels,andallies
Sphenisciformes—penguins
Pelecaniformes—pelicansandallies
Phaethontiformes—tropicbirds
Ciconiiformes—storksandallies
Cathartiformes—NewWorldvultures
Phoenicopteriformes—flamingos
Falconiformes—falcons,Eagles,hawksandallies
Gruiformes—cranesandallies
Pteroclidiformes—sandgrouse
Columbiformes—dovesandpigeons
Psittaciformes—parrotsandallies
Cuculiformes—cuckoosandturacos
Opisthocomiformes—hoatzin
Strigiformes—owls
Caprimulgiformes—nightjarsandallies
Apodiformes—swiftsandhummingbirds
Coraciiformes—kingfishersandallies
Piciformes—woodpeckersandallies
Trogoniformes—trogons
Coliiformes—mousebirds
Passeriformes—passerines
TheradicallydifferentSibley-Monroeclassification(Sibley-Ahlquisttaxonomy),basedonmoleculardata,foundwidespreadadoptioninafewaspects,asrecentmolecular,fossil,andanatomicalevidencesupportedtheGalloanseraeforexample.[24]

Distribution

TherangeoftheHouseSparrowhasexpandeddramaticallyduetohumanactivities.[28]Birdsliveandbreedinmostterrestrialhabitatsandonallsevencontinents,reachingtheirsouthernextremeintheSnowPetrel'sbreedingcoloniesupto440kilometres(270mi)inlandinAntarctica.[29]Thehighestbirddiversityoccursintropicalregions.Itwasearlierthoughtthatthishighdiversitywastheresultofhigherspeciationratesinthetropics,howeverrecentstudiesfoundhigherspeciationratesinthehighlatitudesthatwereoffsetbygreaterextinctionratesthaninthetropics.[30]Severalfamiliesofbirdshaveadaptedtolifebothontheworld'soceansandinthem,withsomeseabirdspeciescomingashoreonlytobreed[31]andsomepenguinshavebeenrecordeddivingupto300metres(980ft).[32]

Manybirdspecieshaveestablishedbreedingpopulationsinareastowhichtheyhavebeenintroducedbyhumans.Someoftheseintroductionshavebeendeliberate;theRing-neckedPheasant,forexample,hasbeenintroducedaroundtheworldasagamebird.[33]Othershavebeenaccidental,suchastheestablishmentofwildMonkParakeetsinseveralNorthAmericancitiesaftertheirescapefromcaptivity.[34]Somespecies,includingCattleEgret,[35]Yellow-headedCaracara[36]andGalah,[37]havespreadnaturallyfarbeyondtheiroriginalrangesasagriculturalpracticescreatedsuitablenewhabitat.

Anatomyandphysiology
Mainarticles:BirdanatomyandBirdvision

Externalanatomyofabird:1Beak,2Head,3Iris,4Pupil,5Mantle,6Lessercoverts,7Scapulars,8Mediancoverts,9Tertials,10Rump,11Primaries,12Vent,13Thigh,14Tibio-tarsalarticulation,15Tarsus,16Feet,17Tibia,18Belly,19Flanks,20Breast,21Throat,22WattleListentothisarticle(info/dl)

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MorespokenarticlesComparedwithothervertebrates,birdshaveabodyplanthatshowsmanyunusualadaptations,mostlytofacilitateflight.

Theskeletonconsistsofverylightweightbones.Theyhavelargeair-filledcavities(calledpneumaticcavities)whichconnectwiththerespiratorysystem.[38]Theskullbonesarefusedanddonotshowcranialsutures.[39]Theorbitsarelargeandseparatedbyabonyseptum.Thespinehascervical,thoracic,lumbarandcaudalregionswiththenumberofcervical(neck)vertebraehighlyvariableandespeciallyflexible,butmovementisreducedintheanteriorthoracicvertebraeandabsentinthelatervertebrae.[40]Thelastfewarefusedwiththepelvistoformthesynsacrum.[39]Theribsareflattenedandthesternumiskeeledfortheattachmentofflightmusclesexceptintheflightlessbirdorders.Theforelimbsaremodifiedintowings.[41]

Likethereptiles,birdsareprimarilyuricotelic,thatis,theirkidneysextractnitrogenouswastesfromtheirbloodstreamandexcreteitasuricacidinsteadofureaorammoniaviatheuretersintotheintestine.Birdsdonothaveaurinarybladderorexternalurethralopeninganduricacidisexcretedalongwithfecesasasemisolidwaste.[42][43]However,birdssuchashummingbirdscanbefacultativelyammonotelic,excretingmostofthenitrogenouswastesasammonia.[44]Theyalsoexcretecreatine,ratherthancreatininelikemammals.[39]Thismaterial,aswellastheoutputoftheintestines,emergesfromthebird'scloaca.[45][46]Thecloacaisamulti-purposeopening:wasteisexpelledthroughit,birdsmatebyjoiningcloaca,andfemaleslayeggsfromit.Inaddition,manyspeciesofbirdsregurgitatepellets.[47]Thedigestivesystemofbirdsisunique,withacropforstorageandagizzardthatcontainsswallowedstonesforgrindingfoodtocompensateforthelackofteeth.[48]Mostbirdsarehighlyadaptedforrapiddigestiontoaidwithflight.[49]Somemigratorybirdshaveadaptedtouseproteinfrommanypartsoftheirbodies,includingproteinfromtheintestines,asadditionalenergyduringmigration.[50]

Birdshaveoneofthemostcomplexrespiratorysystemsofallanimalgroups.[39]Uponinhalation,75%ofthefreshairbypassesthelungsandflowsdirectlyintoaposteriorairsacwhichextendsfromthelungsandconnectswithairspacesinthebonesandfillsthemwithair.Theother25%oftheairgoesdirectlyintothelungs.Whenthebirdexhales,theusedairflowsoutofthelungandthestoredfreshairfromtheposteriorairsacissimultaneouslyforcedintothelungs.Thus,abird'slungsreceiveaconstantsupplyoffreshairduringbothinhalationandexhalation.[51]Soundproductionisachievedusingthesyrinx,amuscularchamberwithseveraltympanicmembraneswhichissituatedatthelowerendofthetrachea,fromwhereitseparates.[52]Thebird'shearthasfourchambersandtherightaorticarchgivesrisetosystemiccirculation(unlikeinthemammalswheretheleftarchisinvolved).[39]Thepostcavareceivesbloodfromthelimbsviatherenalportalsystem.Unlikeinmammals,theredbloodcellsinbirdshaveanucleus.[53]

Thenervoussystemislargerelativetothebird'ssize.[39]Themostdevelopedpartofthebrainistheonethatcontrolstheflight-relatedfunctions,whilethecerebellumcoordinatesmovementandthecerebrumcontrolsbehaviourpatterns,navigation,matingandnestbuilding.Mostbirdshaveapoorsenseofsmellwithnotableexceptionsincludingkiwis,[54]NewWorldvultures[55]andtubenoses.[56]Theavianvisualsystemisusuallyhighlydeveloped.Waterbirdshavespecialflexiblelenses,allowingaccommodationforvisioninairandwater.[39]Somespeciesalsohavedualfovea.Birdsaretetrachromatic,possessingultraviolet(UV)sensitiveconecellsintheeyeaswellasgreen,redandblueones.[57]Thisallowsthemtoperceiveultravioletlight,whichisinvolvedincourtship.Manybirdsshowplumagepatternsinultravioletthatareinvisibletothehumaneye;somebirdswhosesexesappearsimilartothenakedeyearedistinguishedbythepresenceofultravioletreflectivepatchesontheirfeathers.MaleBlueTitshaveanultravioletreflectivecrownpatchwhichisdisplayedincourtshipbyposturingandraisingoftheirnapefeathers.[58]Ultravioletlightisalsousedinforaging—kestrelshavebeenshowntosearchforpreybydetectingtheUVreflectiveurinetrailmarksleftonthegroundbyrodents.[59]Theeyelidsofabirdarenotusedinblinking.Insteadtheeyeislubricatedbythenictitatingmembrane,athirdeyelidthatmoveshorizontally.[60]Thenictitatingmembranealsocoverstheeyeandactsasacontactlensinmanyaquaticbirds.[39]Thebirdretinahasafanshapedbloodsupplysystemcalledthepecten.[39]Mostbirdscannotmovetheireyes,althoughthereareexceptions,suchastheGreatCormorant.[61]Birdswitheyesonthesidesoftheirheadshaveawidevisualfield,whilebirdswitheyesonthefrontoftheirheads,suchasowls,havebinocularvisionandcanestimatethedepthoffield.[62]Theavianearlacksexternalpinnaebutiscoveredbyfeathers,althoughinsomebirds,suchastheAsio,BuboandOtusowls,thesefeathersformtuftswhichresembleears.Theinnerearhasacochlea,butitisnotspiralasinmammals.[63]

Afewspeciesareabletousechemicaldefensesagainstpredators;someProcellariiformescanejectanunpleasantoilagainstanaggressor,[64]andsomespeciesofpitohuisfromNewGuineasecreteapowerfulneurotoxinintheirskinandfeathers.[65]

Chromosomes
Birdshavetwosexes:maleandfemale.Birds'sexisdeterminedbyZandWsexchromosomes,ratherthantheXandYchromosomesseeninmammals.MalescarrytwoZchromosomes(ZZ),andfemalescarryaWchromosomeandaZchromosome(WZ).[39]Innearlyallspecies,anindividual'ssexisdeterminedatfertilization.However,onerecentstudydemonstratedtemperature-dependentsexdeterminationamongAustralianBrush-turkeys,forwhichhighertemperaturesduringincubationresultedinahigherfemale-to-malesexratio.[66]

Feathers,plumage,andscales
Mainarticles:FeatherandFlightfeather

TheplumageoftheAfricanScopsOwlallowsittoblendinwithitssurroundings.Feathersareafeatureuniquetobirds.Theyfacilitateflight,provideinsulationthataidsinthermoregulation,andareusedindisplay,camouflage,andsignaling.[39]Thereareseveraltypesoffeathers,eachservingitsownsetofpurposes.Feathersareepidermalgrowthsattachedtotheskinandariseonlyinspecifictractsofskincalledpterylae.Thedistributionpatternofthesefeathertracts(pterylosis)isusedintaxonomyandsystematics.Thearrangementandappearanceoffeathersonthebody,calledplumage,mayvarywithinspeciesbyage,socialstatus,[67]andsex.[68]

Plumageisregularlymoulted;thestandardplumageofabirdthathasmoultedafterbreedingisknownasthe"non-breeding"plumage,or–intheHumphrey-Parkesterminology–"basic"plumage;breedingplumagesorvariationsofthebasicplumageareknownundertheHumphrey-Parkessystemas"alternate"plumages.[69]Moultingisannualinmostspecies,althoughsomemayhavetwomoultsayear,andlargebirdsofpreymaymoultonlyonceeveryfewyears.Moultingpatternsvaryacrossspecies.Inpasserines,flightfeathersarereplacedoneatatimewiththeinnermostprimarybeingthefirst.Whenthefifthofsixthprimaryisreplaced,theoutermosttertiariesbegintodrop.Aftertheinnermosttertiariesaremoulted,thesecondariesstartingfromtheinnermostbegintodropandthisproceedstotheouterfeathers(centrifugalmoult).Thegreaterprimarycovertsaremoultedinsynchronywiththeprimarythattheyoverlap.[70]Asmallnumberofspecies,suchasducksandgeese,losealloftheirflightfeathersatonce,temporarilybecomingflightless.[71]Asageneralrule,thetailfeathersaremoultedandreplacedstartingwiththeinnermostpair.[70]CentripetalmoultsoftailfeathersarehoweverseeninthePhasianidae.[72]Thecentrifugalmoultismodifiedinthetailfeathersofwoodpeckersandtreecreepers,inthatitbeginswiththesecondinnermostpairoffeathersandfinisheswiththecentralpairoffeatherssothatthebirdmaintainsafunctionalclimbingtail.[70][73]Thegeneralpatternseeninpasserinesisthattheprimariesarereplacedoutward,secondariesinward,andthetailfromcenteroutward.[74]Beforenesting,thefemalesofmostbirdspeciesgainabarebroodpatchbylosingfeathersclosetothebelly.Theskinthereiswellsuppliedwithbloodvesselsandhelpsthebirdinincubation.[75]

RedLorypreeningFeathersrequiremaintenanceandbirdspreenorgroomthemdaily,spendinganaverageofaround9%oftheirdailytimeonthis.[76]Thebillisusedtobrushawayforeignparticlesandtoapplywaxysecretionsfromtheuropygialgland;thesesecretionsprotectthefeathers'flexibilityandactasanantimicrobialagent,inhibitingthegrowthoffeather-degradingbacteria.[77]Thismaybesupplementedwiththesecretionsofformicacidfromants,whichbirdsreceivethroughabehaviourknownasanting,toremovefeatherparasites.[78]

Thescalesofbirdsarecomposedofthesamekeratinasbeaks,claws,andspurs.Theyarefoundmainlyonthetoesandmetatarsus,butmaybefoundfurtherupontheankleinsomebirds.Mostbirdscalesdonotoverlapsignificantly,exceptinthecasesofkingfishersandwoodpeckers.Thescalesofbirdsarethoughttobehomologoustothoseofreptilesandmammals.[79]

Flight
Mainarticle:Birdflight

RestlessFlycatcherinthedownstrokeofflappingflightMostbirdscanfly,whichdistinguishesthemfromalmostallothervertebrates.Flightistheprimarymeansoflocomotionformostbirdspeciesandisusedforbreeding,feeding,andpredatoravoidanceandescape.Birdshavevariousadaptationsforflight,includingalightweightskeleton,twolargeflightmuscles(thepectoralis—accountingfor15%ofthetotalmassofthebird—andthesupracoracoideus),andamodifiedforelimb(wing)thatservesasanaerofoil.[39]Wingshapeandsizegenerallydetermineabirdspecies'typeofflight;manybirdscombinepowered,flappingflightwithlessenergy-intensivesoaringflight.About60extantbirdspeciesareflightless,asweremanyextinctbirds.[80]Flightlessnessoftenarisesinbirdsonisolatedislands,probablyduetolimitedresourcesandtheabsenceoflandpredators.[81]Thoughflightless,penguinsusesimilarmusculatureandmovementsto"fly"throughthewater,asdoauks,shearwatersanddippers.[82]

Behaviour
Mostbirdsarediurnal,butsomebirds,suchasmanyspeciesofowlsandnightjars,arenocturnalorcrepuscular(activeduringtwilighthours),andmanycoastalwadersfeedwhenthetidesareappropriate,bydayornight.[83]

Dietandfeeding

FeedingadaptationsinbeaksBirds'dietsarevariedandoftenincludenectar,fruit,plants,seeds,carrion,andvarioussmallanimals,includingotherbirds.[39]Becausebirdshavenoteeth,theirdigestivesystemisadaptedtoprocessunmasticatedfooditemsthatareswallowedwhole.

Birdsthatemploymanystrategiestoobtainfoodorfeedonavarietyoffooditemsarecalledgeneralists,whileothersthatconcentratetimeandeffortonspecificfooditemsorhaveasinglestrategyto

big bird

obtainfoodareconsideredspecialists.[39]Birds'feedingstrategiesvarybyspecies.Manybirdsgleanforinsects,invertebrates,fruit,orseeds.Somehuntinsectsbysuddenlyattackingfromabranch.Nectarfeederssuchashummingbirds,sunbirds,lories,andlorikeetsamongstothershavespeciallyadaptedbrushytonguesandinmanycasesbillsdesignedtofitco-adaptedflowers.[84]Kiwisandshorebirdswithlongbillsprobeforinvertebrates;shorebirds'variedbilllengthsandfeedingmethodsresultintheseparationofecologicalniches.[39][85]Loons,divingducks,penguinsandaukspursuetheirpreyunderwater,usingtheirwingsorfeetforpropulsion,[31]whileaerialpredatorssuchassulids,kingfishersandternsplungediveaftertheirprey.Flamingos,threespeciesofprion,andsomeducksarefilterfeeders.[86][87]Geeseanddabblingducksareprimarilygrazers.Somespecies,includingfrigatebirds,gulls,[88]andskuas,[89]engageinkleptoparasitism,stealingfooditemsfromotherbirds.Kleptoparasitismisthoughttobeasupplementtofoodobtainedbyhunting,ratherthanasignificantpartofanyspecies'diet;astudyofGreatFrigatebirdsstealingfromMaskedBoobiesestimatedthatthefrigatebirdsstoleatmost40%oftheirfoodandonaveragestoleonly5%.[90]Otherbirdsarescavengers;someofthese,likevultures,arespecialisedcarrioneaters,whileothers,likegulls,corvids,orotherbirdsofprey,areopportunists.[91]

Migration
Mainarticle:Birdmigration
Manybirdspeciesmigratetotakeadvantageofglobaldifferencesofseasonaltemperatures,thereforeoptimisingavailabilityoffoodsourcesandbreedinghabitat.Thesemigrationsvaryamongthedifferentgroups.Manylandbirds,shorebirds,andwaterbirdsundertakeannuallongdistancemigrations,usuallytriggeredbythelengthofdaylightaswellasweatherconditions.Thesebirdsarecharacterisedbyabreedingseasonspentinthetemperateorarctic/antarcticregionsandanon-breedingseasoninthetropicalregionsoroppositehemisphere.Beforemigration,birdssubstantiallyincreasebodyfatsandreservesandreducethesizeofsomeoftheirorgans.[92][50]Migrationishighlydemandingenergetically,particularlyasbirdsneedtocrossdesertsandoceanswithoutrefuelling.Landbirdshaveaflightrangeofaround2,500km(1,600mi)andshorebirdscanflyupto4,000km(2,500mi),[39]althoughtheBar-tailedGodwitiscapableofnon-stopflightsofupto10,200km(6,300mi).[93]Seabirdsalsoundertakelongmigrations,t

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helongestannualmigrationbeingthoseofSootyShearwaters,whichnestinNewZealandandChileandspendthenorthernsummerfeedingintheNorthPacificoffJapan,AlaskaandCalifornia,anannualroundtripof64,000km(39,800mi).[94]Otherseabirdsdisperseafterbreeding,travellingwidelybuthavingnosetmigrationroute.AlbatrossesnestingintheSouthernOceanoftenundertakecircumpolartripsbetweenbreedingseasons.[95]

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TheroutesofsatellitetaggedBar-tailedGodwitsmigratingnorthfromNewZealand.Thisspecieshasthelongestknownnon-stopmigrationofanyspecies,upto10,200km(6,300mi).Somebirdspeciesundertakeshortermigrations,travellingonlyasfarasisrequiredtoavoidbadweatherorobtainfood.Irruptivespeciessuchastheborealfinchesareonesuchgroupandcancommonlybefoundatalocationinoneyearandabsentthenext.Thistypeofmigrationisnormallyassociatedwithfoodavailability.[96]Speciesmayalsotravelshorterdistancesoverpartoftheirrange,withindividualsfromhigherlatitudestravellingintotheexistingrangeofconspecifics;ot

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hersundertakepartialmigrations,whereonlyafractionofthepopulation,usuallyfemalesandsubdominantmales,migrates.[97]Partialmigrationcanformalargepercentageofthemigrationbehaviourofbirdsinsomeregions;inAustralia,surveysfoundthat44%ofnon-passerinebirdsand32%ofpasserineswerepartiallymigratory.[98]Altitudinalmigrationisaformofshortdistancemigrationinwhichbirdsspendthebreedingseasonathigheraltitudeselevationsandmovetoloweronesduringsuboptimalconditions.Itismostoftentriggeredbytempera

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turechangesandusuallyoccurswhenthenormalterritoriesalsobecomeinhospitableduetolackoffood.[99]Somespeciesmayalsobenomadic,holdingnofixedterritoryandmovingaccordingtoweatherandfoodavailability.Parrotsasafamilyareoverwhelminglyneithermigratorynorsedentarybutconsideredtoeither