Editing Bird (section)

==Behaviour==
Most birds are [[diurnal animal|diurnal]], but some birds, such as many species of [[owl]]s and [[nightjar]]s, are [[nocturnal]] or [[crepuscular]] (active during twilight hours), and many coastal [[wader]]s feed when the tides are appropriate, by day or night.<ref>{{Cite journal|last1=Robert |first1=Michel |date=January 1989 |title=Conditions and significance of night feeding in shorebirds and other water birds in a tropical lagoon |journal=The Auk |volume=106 |issue=1 |pages=94–101 |doi=10.2307/4087761 |last2=McNeil |first2=Raymond |last3=Leduc |first3=Alain |jstor=4087761 |url=http://sora.unm.edu/sites/default/files/journals/auk/v106n01/p0094-p0101.pdf |archive-url=https://web.archive.org/web/20141004070208/http://sora.unm.edu/sites/default/files/journals/auk/v106n01/p0094-p0101.pdf |archive-date=4 October 2014}}</ref>

===Diet and feeding===
{{see also|Bird food}}
[[File:BirdBeaksA.svg|thumb|upright|right|alt= Illustration of the heads of 16 types of birds with different shapes and sizes of beak|Feeding adaptations in beaks]]
{{Birdgloss|dietary classification terms (-vores)|Birds' diets}} are varied and often include [[nectar (plant)|nectar]], fruit, plants, seeds, [[carrion]], and various small animals, including other birds.<ref name="Gill"/> The [[digestive system of birds]] is unique, with a [[Crop (anatomy)|crop]] for storage and a [[gizzard]] that contains swallowed stones for grinding food to compensate for the lack of teeth.<ref>{{Cite journal|last=Gionfriddo |first=James P.|author2=Best|date=1 February 1995|title=Grit Use by House Sparrows: Effects of Diet and Grit Size|journal=Condor|volume=97|issue=1|pages=57–67|doi=10.2307/1368983 |jstor=1368983 |url=http://sora.unm.edu/sites/default/files/journals/condor/v097n01/p0057-p0067.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://sora.unm.edu/sites/default/files/journals/condor/v097n01/p0057-p0067.pdf |archive-date=9 October 2022 |url-status=live}}</ref> Some species such as pigeons and some psittacine species do not have a [[gallbladder]].<ref>{{Cite journal |date=2010|title=Complex Evolution of Bile Salts in Birds|journal=The Auk|doi=10.1525/auk.2010.09155 |pmc=2990222|last1=Hagey|first1=Lee R.|last2=Vidal|first2=Nicolas|last3=Hofmann|first3=Alan F. |last4=Krasowski|first4=Matthew D.|volume=127|issue=4 |pages=820–831|pmid=21113274}}</ref> Most birds are highly adapted for rapid digestion to aid with flight.<ref>{{Cite book|last=Attenborough|first=David |title=The Life of Birds|title-link=The Life of Birds |publisher=Princeton University Press|year=1998 |isbn=0-691-01633-X|location=Princeton|author-link=David Attenborough}}</ref> Some migratory birds have adapted to use protein stored in many parts of their bodies, including protein from the intestines, as additional energy during migration.<ref name="Battley2">{{Cite journal|last1=Battley|first1=Phil F. |last2=Piersma|first2=T|last3=Dietz|first3=MW|last4=Tang|first4=S|last5=Dekinga|first5=A|last6=Hulsman |first6=K|date=January 2000|title=Empirical evidence for differential organ reductions during trans-oceanic bird flight|journal=[[Proceedings of the Royal Society B]]|volume=267|issue=1439|pages=191–195 |doi=10.1098/rspb.2000.0986|pmc=1690512 |pmid=10687826}} (Erratum in ''Proceedings of the Royal Society B'' '''267'''(1461):2567.)</ref>

Birds that employ many strategies to obtain food or feed on a variety of food items are called generalists, while others that concentrate time and effort on specific food items or have a single strategy to obtain food are considered specialists.<ref name="Gill"/> [[Avian foraging]] strategies can vary widely by species. Many birds [[Gleaning (birds)|glean]] for insects, invertebrates, fruit, or seeds. Some hunt insects by suddenly attacking from a branch. Those species that seek [[Pest (organism)|pest]] [[insect]]s are considered beneficial 'biological control agents' and their presence encouraged in [[biological pest control]] programmes.<ref name="lwa001">{{cite web |url=http://lwa.gov.au/files/products/land-water-and-wool/pf061365/pf061365.pdf |title=Birds on New England wool properties – A woolgrower guide |access-date=17 July 2010 |publisher=Australian Government – Land and Water Australia |work=Land, Water & Wool Northern Tablelands Property Fact Sheet |author=Reid, N. |year=2006 |archive-date=15 March 2011 |archive-url=https://web.archive.org/web/20110315005353/http://lwa.gov.au/files/products/land-water-and-wool/pf061365/pf061365.pdf}}</ref> Combined, insectivorous birds eat 400–500 million metric tons of arthropods annually.<ref>{{cite journal |last1=Nyffeler |first1=M. |last2=Şekercioğlu |first2=Ç. H. |last3=Whelan |first3=C. J. |date=August 2018 |title=Insectivorous birds consume an estimated 400–500 million tons of prey annually |journal=[[The Science of Nature]] |volume=105 |issue=7–8 |page= 47|doi=10.1007/s00114-018-1571-z |pmid=29987431 |pmc=6061143 |bibcode=2018SciNa.105...47N }}</ref>

Nectar feeders such as [[hummingbird]]s, [[sunbird]]s, [[lories and lorikeets|lories, and lorikeets]] amongst others have specially adapted brushy tongues and in many cases bills designed to fit [[Coevolution|co-adapted]] flowers.<ref>{{Cite journal|last1=Paton |first1=D. C. |date=1 April 1989|title=Bills and tongues of nectar-feeding birds: A review of morphology, function, and performance, with intercontinental comparisons |journal=Australian Journal of Ecology |volume=14 |issue=4 |pages=473–506 |doi=10.1111/j.1442-9993.1989.tb01457.x |first2=B.G. |last2=Collins |bibcode=1989AusEc..14..473P }}</ref> [[Kiwi (bird)|Kiwi]]s and [[shorebird]]s with long bills probe for invertebrates; shorebirds' varied bill lengths and feeding methods result in the separation of [[ecological niche]]s.<ref name="Gill"/><ref>{{Cite journal|last1=Baker |first1=Myron Charles |date=1 April 1973|title=Niche Relationships Among Six Species of Shorebirds on Their Wintering and Breeding Ranges |journal=Ecological Monographs |volume=43 |issue=2 |pages=193–212 |doi=10.2307/1942194 |first2=Ann Eileen Miller |last2=Baker |jstor=1942194|bibcode=1973EcoM...43..193B }}</ref> [[Loon|Diver]]s, [[diving duck]]s, [[penguin]]s and [[auks]] pursue their prey underwater, using their wings or feet for propulsion,<ref name="Burger"/> while aerial predators such as [[sulidae|sulids]], [[kingfisher]]s and [[tern]]s plunge dive after their prey. [[Flamingo]]s, three species of [[prion (bird)|prion]], and some ducks are [[filter feeder]]s.<ref>{{Cite journal|last1=Cherel |first1=Yves |year=2002 |title=Food and feeding ecology of the sympatric thin-billed ''Pachyptila belcheri'' and Antarctic ''P. desolata'' prions at Iles Kerguelen, Southern Indian Ocean |journal=Marine Ecology Progress Series |volume=228 |pages=263–281 |doi=10.3354/meps228263 |last2=Bocher |first2=P |last3=De Broyer |first3=C |last4=Hobson |first4=KA|bibcode=2002MEPS..228..263C |doi-access=free }}</ref><ref>{{Cite journal|last=Jenkin |first=Penelope M. |year=1957 |title=The Filter-Feeding and Food of Flamingoes (Phoenicopteri) |journal=Philosophical Transactions of the Royal Society B |volume=240 |issue=674 |pages=401–493 |doi=10.1098/rstb.1957.0004 |jstor=92549|bibcode=1957RSPTB.240..401J }}</ref> [[Geese]] and [[dabbling duck]]s are primarily grazers.<ref>{{cite book |last1=Hughes |first1=Baz |last2=Green |first2=Andy J. |chapter=Feeding Ecology |editor-last1=Kear |editor-first1= Janet|title=Ducks, Geese and Swans. |publisher=Oxford University Press |year=2005|pages=42–44 |isbn=978-0-19-861008-3 |chapter-url=https://books.google.com/books?id=MfrdBcKd79wC&pg=PA42}}</ref><ref>{{cite journal |title=The Craniolingual Morphology of Waterfowl (Aves, Anseriformes) and Its Relationship with Feeding Mode Revealed Through Contrast-Enhanced X-Ray Computed Tomography and 2D Morphometrics |year=2016 |last1=Li |first1=Zhiheng |last2=Clarke |first2=Julia A. |journal=Evolutionary Biology |volume=43 |issue=1 |pages=12–25 |doi=10.1007/s11692-015-9345-4 |bibcode=2016EvBio..43...12L }}</ref>

Some species, including [[frigatebird]]s, [[gull]]s,<ref>{{cite journal |last1=Takahashi |first1=Akinori |last2=Kuroki |first2=Maki |last3=Niizuma |first3=Yasuaki |last4=Watanuki |first4=Yutaka |title=Parental Food Provisioning Is Unrelated to Manipulated Offspring Food Demand in a Nocturnal Single-Provisioning Alcid, the Rhinoceros Auklet |journal=Journal of Avian Biology |date=December 1999 |volume=30 |issue=4 |pages=486 |doi=10.2307/3677021 |jstor=3677021 }}</ref> and [[skua]]s,<ref>{{Cite journal|last=Bélisle |first=Marc |date=1 August 1995|title=Predation and kleptoparasitism by migrating Parasitic Jaegers |journal=The Condor |volume=97 |issue=3 |pages=771–781 |doi=10.2307/1369185 |url=http://sora.unm.edu/sites/default/files/journals/condor/v097n03/p0771-p0781.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://sora.unm.edu/sites/default/files/journals/condor/v097n03/p0771-p0781.pdf |archive-date=9 October 2022 |url-status=live|author2=Giroux|jstor=1369185 }}</ref> engage in [[kleptoparasitism]], stealing food items from other birds. Kleptoparasitism is thought to be a supplement to food obtained by hunting, rather than a significant part of any species' diet; a study of [[great frigatebird]]s stealing from [[masked booby|masked boobies]] estimated that the frigatebirds stole at most 40% of their food and on average stole only 5%.<ref>{{cite journal |last1=Vickery |first1=J. A. |title=The Kleptoparasitic Interactions between Great Frigatebirds and Masked Boobies on Henderson Island, South Pacific |url=https://archive.org/details/sim_condor_1994-05_96_2/page/331 |journal=The Condor |date=May 1994 |volume=96 |issue=2 |pages=331–340 |doi=10.2307/1369318 |jstor=1369318 }}</ref> Other birds are [[scavenger]]s; some of these, like [[vulture]]s, are specialised carrion eaters, while others, like gulls, [[corvid]]s, or other birds of prey, are opportunists.<ref>{{Cite journal|last1=Hiraldo |first1=F. C. |year=1991 |title=Unspecialized exploitation of small carcasses by birds |journal=Bird Studies |volume=38 |issue=3 |pages=200–207 |doi=10.1080/00063659109477089 |last2=Blanco |first2=J. C. |last3=Bustamante |first3=J.|bibcode=1991BirdS..38..200H |hdl=10261/47141 |hdl-access=free }}</ref>

===Water and drinking===
Water is needed by many birds although their mode of excretion and lack of [[sweat gland]]s reduces the physiological demands.<ref>{{Cite book|year=2005|url=http://irs.ub.rug.nl/ppn/287916626|isbn=90-367-2378-7 |last=Engel|first=Sophia Barbara|title=Racing the wind: Water economy and energy expenditure in avian endurance flight|publisher=University of Groningen|access-date=25 November 2008|archive-date=5 April 2020|archive-url=https://web.archive.org/web/20200405201429/http://irs.ub.rug.nl/ppn/287916626}}</ref> Some desert birds can obtain their water needs entirely from moisture in their food. Some have other adaptations such as allowing their body temperature to rise, saving on moisture loss from evaporative cooling or panting.<ref>{{cite journal |last1=Tieleman |first1=B. Irene |last2=Williams |first2=Joseph B. |title=The Role of Hyperthermia in the Water Economy of Desert Birds |journal=Physiological and Biochemical Zoology |date=January 1999 |volume=72 |issue=1 |pages=87–100 |doi=10.1086/316640 |pmid=9882607 |hdl= 11370/6edc6940-c2e8-4c96-832e-0b6982dd59c1 |url=https://pure.rug.nl/ws/files/62402849/The_Role_of_Hyperthermia_in_the_Water_Economy_of_Desert_Birds.pdf |hdl-access=free }}</ref> Seabirds can drink seawater and have [[salt gland]]s inside the head that eliminate excess salt out of the nostrils.<ref>{{Cite journal|title=The Salt-Secreting Gland of Marine Birds|last=Schmidt-Nielsen|first=Knut|journal=Circulation|date=1 May 1960|volume=21|pages=955–967|issue=5|doi=10.1161/01.CIR.21.5.955|pmid=14443123 |doi-access=free}}</ref>

Most birds scoop water in their beaks and raise their head to let water run down the throat. Some species, especially of arid zones, belonging to the [[Columbidae|pigeon]], [[Estrildidae|finch]], [[Coliidae|mousebird]], [[Turnicidae|button-quail]] and [[Otididae|bustard]] families are capable of sucking up water without the need to tilt back their heads.<ref>{{Cite journal|first=Sara L.|last=Hallager|title=Drinking methods in two species of bustards|journal=Wilson Bull.|volume=106|issue=4|year=1994|pages=763–764|hdl=10088/4338}}</ref> Some desert birds depend on water sources and [[sandgrouse]] are particularly well known for congregating daily at waterholes. Nesting sandgrouse and many plovers carry water to their young by wetting their belly feathers.<ref>{{Cite journal|title=Water Transport by Sandgrouse|url=https://archive.org/details/sim_bioscience_1983-06_33_6/page/365|first=Gordon L.|last= MacLean|journal=BioScience|volume=33|issue= 6|date=1 June 1983|pages=365–369|doi=10.2307/1309104|jstor=1309104}}</ref> Some birds carry water for chicks at the nest in their crop or regurgitate it along with food. The pigeon family, flamingos and penguins have adaptations to produce a nutritive fluid called [[crop milk]] that they provide to their chicks.<ref>{{cite journal|author=Eraud C|author2=Dorie A|author3=Jacquet A|author4=Faivre B|year=2008|title= The crop milk: a potential new route for carotenoid-mediated parental effects| journal= Journal of Avian Biology| volume=39| pages= 247–251| doi= 10.1111/j.0908-8857.2008.04053.x|issue=2|url=https://hal.archives-ouvertes.fr/hal-00294461/file/Eraud2008.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://hal.archives-ouvertes.fr/hal-00294461/file/Eraud2008.pdf |archive-date=9 October 2022 |url-status=live}}</ref>

===Feather care===
{{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 |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 |doi-access=free}}</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===
{{Main|Bird migration}}
[[File:CanadianGeeseFlyingInVFormation.jpg|thumb|A flock of [[Canada geese]] in [[V formation]]]]
Many bird species migrate to take advantage of global differences of [[season]]al temperatures, therefore optimising availability of food sources and breeding habitat. These migrations vary among the different groups. Many landbirds, [[shorebird]]s, and [[waterbird]]s undertake annual long-distance migrations, usually triggered by the length of daylight as well as weather conditions. These birds are characterised by a breeding season spent in the [[temperate]] or [[polar region]]s and a non-breeding season in the [[tropical]] regions or opposite hemisphere. Before migration, birds substantially increase body fats and reserves and reduce the size of some of their organs.<ref name="Battley">{{Cite journal|last1=Battley |first1=Phil F. |date=January 2000 |title=Empirical evidence for differential organ reductions during trans-oceanic bird flight |journal=[[Proceedings of the Royal Society B]] |volume=267 |issue=1439 |pages=191–195 |doi=10.1098/rspb.2000.0986 |pmid=10687826 |last2=Piersma |first2=T. |last3=Dietz |first3=M. W.|last4=Tang |first4=S |last5=Dekinga |first5=A. |last6=Hulsman |first6=K. |pmc=1690512}} (Erratum in ''Proceedings of the Royal Society B'' '''267'''(1461):2567.)</ref><ref name="Klaassen">{{Cite journal|last=Klaassen |first=Marc |date=1 January 1996|title=Metabolic constraints on long-distance migration in birds |journal=Journal of Experimental Biology |volume=199 |issue=1 |pages=57–64 |doi=10.1242/jeb.199.1.57 |pmid=9317335 |bibcode=1996JExpB.199...57K |url=http://jeb.biologists.org/cgi/reprint/199/1/57 }}</ref>

Migration is highly demanding energetically, particularly as birds need to cross deserts and oceans without refuelling. Landbirds have a flight range of around {{convert|2500|km|mi|-2|abbr=on}} and shorebirds can fly up to {{convert|4000|km|mi|-2|abbr=on}},<ref name="Gill"/> although the [[bar-tailed godwit]] is capable of non-stop flights of up to {{convert|10200|km|mi|-2|abbr=on}}.<ref>{{Cite news |title=Long-distance Godwit sets new record |url=http://www.birdlife.org/news/news/2007/04/bar-tailed_godwit_journey.html |date=4 May 2007 |publisher=[[BirdLife International]] |access-date=13 December 2007 |archive-date=2 October 2013 |archive-url=https://web.archive.org/web/20131002131732/http://www.birdlife.org/news/news/2007/04/bar-tailed_godwit_journey.html }}</ref> Some [[seabird]]s undertake long migrations, with the longest annual migrations including those of [[Arctic terns]], which were recorded travelling an average of {{convert|70900|km|abbr=on}} between their [[Arctic]] breeding grounds in [[Greenland]] and [[Iceland]] and their wintering grounds in [[Antarctica]], with one bird covering {{convert|81600|km|abbr=on}},<ref>{{cite journal |last1=Egevang |first1=Carsten |last2=Stenhouse |first2=Iain J. |last3=Phillips |first3=Richard A. |last4=Petersen |first4=Aevar |last5=Fox |first5=James W. |last6=Silk |first6=Janet R. D. |year=2010 |title=Tracking of Arctic terns ''Sterna paradisaea'' reveals longest animal migration |journal=Proceedings of the National Academy of Sciences |volume=107 |issue=5 |pages=2078–2081 |doi=10.1073/pnas.0909493107 |doi-access=free |pmid=20080662 |pmc=2836663 |bibcode=2010PNAS..107.2078E }}</ref> and [[sooty shearwater]]s, which nest in [[New Zealand]] and [[Chile]] and make annual round trips of {{convert|64000|km|mi|-2|abbr=on}} to their summer feeding grounds in the North Pacific off Japan, [[Alaska]] and [[California]].<ref>{{Cite journal|last1=Shaffer |first1=Scott A. |year=2006 |title=Migratory shearwaters integrate oceanic resources across the Pacific Ocean in an endless summer |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=103 |issue=34 |pages=12799–12802 |doi=10.1073/pnas.0603715103 |pmid= 16908846 |last2=Tremblay |first2=Y.. |last3=Weimerskirch |first3=H. |last4=Scott |first4=D |last5=Thompson |first5=DR |last6=Sagar |first6=PM |last7=Moller |first7=H. |last8=Taylor |first8=G. A. |last9=Foley |first9=D. G.|pmc=1568927|first10=B. A.|last10=Block|first11=D. P.|last11=Costa|display-authors=1 |bibcode=2006PNAS..10312799S |doi-access=free }}</ref>
Other seabirds disperse after breeding, travelling widely but having no set migration route. [[Albatross]]es nesting in the [[Southern Ocean]] often undertake circumpolar trips between breeding seasons.<ref>{{Cite journal|last1=Croxall |first1=John P. |year=2005 |title=Global Circumnavigations: Tracking year-round ranges of nonbreeding Albatrosses |journal=Science |volume=307 |issue=5707 |pages=249–250 |doi=10.1126/science.1106042 |pmid=15653503 |last2=Silk |first2=J. R. |last3=Phillips |first3=R. A. |last4=Afanasyev |first4=V. |last5=Briggs |first5=D. R.|bibcode=2005Sci...307..249C }}</ref>

[[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 }}</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 |doi-access=free }}</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|bibcode=2001EmuAO.101..281C }}</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 |archive-url=https://ghostarchive.org/archive/20221009/http://sora.unm.edu/sites/default/files/journals/auk/v102n04/p0805-p0819.pdf |archive-date=9 October 2022 |url-status=live}}</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 |series=Vol. 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>

The ability of birds to return to precise locations across vast distances has been known for some time; in an experiment conducted in the 1950s, a [[Manx shearwater]] released in [[Boston]] in the United States returned to its colony in [[Skomer]], in Wales within 13 days, a distance of {{convert|5150|km|mi|-2|abbr=on}}.<ref>{{Cite journal|last=Matthews |first=G.V.T. |date=1 September 1953 |title=Navigation in the Manx Shearwater |journal=Journal of Experimental Biology |volume=30 |issue=2 |pages=370–396 |doi=10.1242/jeb.30.3.370 |doi-access=free|bibcode=1953JExpB..30..370M }}</ref> Birds navigate during migration using a variety of methods. For [[diurnal animal|diurnal]] migrants, the [[sun]] is used to navigate by day, and a stellar compass is used at night. Birds that use the sun compensate for the changing position of the sun during the day by the use of an [[Chronobiology|internal clock]].<ref name="Gill"/> Orientation with the stellar compass depends on the position of the [[constellation]]s surrounding [[Polaris]].<ref>{{Cite journal|last1=Mouritsen |first1=Henrik |last2=Larsen |first2=Ole Næsbye |date=15 November 2001 |title=Migrating songbirds tested in computer-controlled Emlen funnels use stellar cues for a time-independent compass |journal=Journal of Experimental Biology |volume=204 |issue=8 |pages=3855–3865 |pmid=11807103 |doi=10.1242/jeb.204.22.3855 |doi-access=free|bibcode=2001JExpB.204.3855M }}</ref> These are backed up in some species by their ability to sense the Earth's [[geomagnetism]] through specialised [[Photoreceptor cell|photoreceptors]].<ref>{{Cite journal|last1=Deutschlander |first1=Mark E. |last2=Phillips |first2=J. B. |last3=Borland |first3=S. C. |date=15 April 1999 |title=The case for light-dependent magnetic orientation in animals |journal=Journal of Experimental Biology |volume=202 |issue=8 |pages=891–908 |pmid=10085262 |doi=10.1242/jeb.202.8.891 |doi-access=free|bibcode=1999JExpB.202..891D }}</ref>

===Communication===
{{See also|Bird vocalisation}}
{{Listen|filename=Troglodytes aedon - House Wren - XC79974.ogg|title=Bird song|description=Song of the [[Northern house wren|house wren]], a common North American songbird
|filename2=Tooth-billed_Catbird_audio09.ogg|title2=Mimicry|description2=A [[tooth-billed bowerbird]] mimicking a [[spangled drongo]]
|filename3=Picidae pecking on wood.ogg|title3=Drumming|description3=A [[woodpecker]] drumming on wood}}Birds [[Animal communication|communicate]] primarily using visual and auditory signals. Signals can be interspecific (between species) and intraspecific (within species).

Birds sometimes use plumage to assess and assert social dominance,<ref>{{Cite journal|last=Möller |first=Anders Pape |year=1988 |title=Badge size in the house sparrow ''Passer domesticus''|journal=[[Behavioral Ecology and Sociobiology]] |volume=22 |issue=5 |pages=373–378 |doi=10.1007/BF00295107 |url=https://www.researchgate.net/publication/226147226 |jstor=4600164}}</ref> to display breeding condition in sexually selected species, or to make threatening displays, as in the [[sunbittern]]'s mimicry of a large predator to ward off [[hawk]]s and protect young chicks.<ref>{{Cite journal|last=Thomas |first=Betsy Trent |date=1 August 1990 |title=Nesting Behavior of Sunbitterns (''Eurypyga helias'') in Venezuela |journal=The Condor |volume=92 |issue=3 |pages=576–581 |doi=10.2307/1368675 |url=http://sora.unm.edu/sites/default/files/journals/condor/v092n03/p0576-p0581.pdf |archive-url=https://web.archive.org/web/20160305194240/http://sora.unm.edu/sites/default/files/journals/condor/v092n03/p0576-p0581.pdf |archive-date=5 March 2016 |author2=Strahl |jstor=1368675 }}</ref>

[[File:Stavenn Eurypiga helias 00.jpg|thumb|left|alt=Large brown patterned ground bird with outstretched wings each with a large spot in the centre|The startling display of the [[sunbittern]] mimics a large predator.]]Visual communication among birds may also involve ritualised displays, which have developed from non-signalling actions such as preening, the adjustments of feather position, pecking, or other behaviour. These displays may signal aggression or submission or may contribute to the formation of pair-bonds.<ref name = "Gill"/> The most elaborate displays occur during courtship, where "dances" are often formed from complex combinations of many possible component movements;<ref>{{Cite journal|last=Pickering |first=S. P. C. |year=2001 |title=Courtship behaviour of the Wandering Albatross ''Diomedea exulans'' at Bird Island, South Georgia |journal=Marine Ornithology |volume=29 |issue=1 |pages=29–37 |url=http://www.marineornithology.org/PDF/29_1/29_1_6.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.marineornithology.org/PDF/29_1/29_1_6.pdf |archive-date=9 October 2022 |url-status=live}}</ref> males' breeding success may depend on the quality of such displays.<ref>{{Cite journal|last=Pruett-Jones |first=S.G. |date=1 May 1990|title=Sexual Selection Through Female Choice in Lawes' Parotia, A Lek-Mating Bird of Paradise |url=https://archive.org/details/sim_evolution_1990-05_44_3/page/486 |journal=[[Evolution (journal)|Evolution]] |volume=44 |issue=3 |pages=486–501 |doi=10.2307/2409431 |author2=Pruett-Jones|jstor=2409431 |pmid=28567971 }}</ref>

[[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 |last1=Genevois |first1=F. |last2=Bretagnolle |first2=V. |title=Male blue petrels reveal their body mass when calling |journal=Ethology Ecology & Evolution |date=September 1994 |volume=6 |issue=3 |pages=377–383 |doi=10.1080/08927014.1994.9522988 |bibcode=1994EtEcE...6..377G }}</ref> bond formation, the claiming and maintenance of territories,<ref name = "Gill"/><ref name=":0" /> 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 }}</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 }}</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 |doi=10.1650/0010-5422(2001)103[0143:DWDEIF]2.0.CO;2 |doi-access=free}}</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===
[[File:Red-billed quelea flocking at waterhole.jpg|thumb|right|alt= massive flock of tiny birds seen from distance so that birds appear as specks|[[Red-billed quelea]]s, the most numerous species of wild bird,<ref name = "flycatcher">{{Cite book |last=Sekercioglu |first=Cagan Hakki |year=2006 |chapter=Foreword |title=Handbook of the Birds of the World |series=Vol. 11: Old World Flycatchers to Old World Warblers |editor=Josep del Hoyo |editor2=Andrew Elliott |editor3=David Christie |location=Barcelona |publisher=Lynx Edicions |isbn=84-96553-06-X |page=48|title-link=Handbook of the Birds of the World}}</ref> form enormous flocks{{snd}}sometimes tens of thousands strong.]]
While some birds are essentially territorial or live in small family groups, other birds may form large [[flock (birds)|flocks]]. The principal benefits of flocking are [[safety in numbers]] and increased foraging efficiency.<ref name = "Gill"/> Defence against predators is particularly important in closed habitats like forests, where [[ambush predation]] is common and multiple eyes can provide a valuable [[early warning system]]. This has led to the development of many [[mixed-species feeding flock]]s, which are usually composed of small numbers of many species; these flocks provide safety in numbers but increase potential competition for resources.<ref>{{Cite journal|last=Terborgh |first=John |year=2005 |title=Mixed flocks and polyspecific associations: Costs and benefits of mixed groups to birds and monkeys |journal=American Journal of Primatology |volume=21 |issue=2|pages=87–100 |doi=10.1002/ajp.1350210203|pmid=31963979 }}</ref> Costs of flocking include bullying of socially subordinate birds by more dominant birds and the reduction of feeding efficiency in certain cases.<ref>{{Cite journal|last=Hutto |first=Richard L. |date=January 1988|title=Foraging Behavior Patterns Suggest a Possible Cost Associated with Participation in Mixed-Species Bird Flocks |journal=[[Oikos (journal)|Oikos]] |volume=51 |issue=1 |pages=79–83 |doi=10.2307/3565809 |jstor=3565809|bibcode=1988Oikos..51...79H }}</ref> Some species have a mixed system with breeding pairs maintaining territories, while unmated or young birds live in flocks where they secure mates prior to finding territories.<ref>{{Cite journal |last1=Sundar |first1=K. S. Gopi |last2=Grant |first2=John D. A. |last3=Veltheim |first3=Inka |last4=Kittur |first4=Swati |last5=Brandis |first5=Kate |last6=McCarthy |first6=Michael A. |last7=Scambler |first7=Elinor |date=2019 |title=Sympatric cranes in northern Australia: abundance, breeding success, habitat preference and diet |journal=Emu - Austral Ornithology |volume=119 |issue=1 |pages=79–89 |doi=10.1080/01584197.2018.1537673 |bibcode=2019EmuAO.119...79S }}</ref>

Birds sometimes also form associations with non-avian species. Plunge-diving [[seabird]]s associate with [[dolphin]]s and [[tuna]], which push shoaling fish towards the surface.<ref name = "AU">{{Cite journal|last=Au |first=David W.K. |date=1 August 1986|title=Seabird interactions with Dolphins and Tuna in the Eastern Tropical Pacific |journal=The Condor |volume=88 |issue=3 |pages=304–317 |url=http://sora.unm.edu/sites/default/files/journals/condor/v088n03/p0304-p0317.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://sora.unm.edu/sites/default/files/journals/condor/v088n03/p0304-p0317.pdf |archive-date=9 October 2022 |url-status=live |doi=10.2307/1368877|author2=Pitman|jstor=1368877 }}</ref> Some species of [[hornbill]]s have a [[Mutualism (biology)|mutualistic relationship]] with [[dwarf mongoose]]s, in which they forage together and warn each other of nearby [[birds of prey]] and other predators.<ref>{{Cite journal|last1=Anne |first1=O. |date=June 1983 |title=Dwarf mongoose and hornbill mutualism in the Taru desert, Kenya |journal=Behavioral Ecology and Sociobiology |volume=12 |issue=3 |pages=181–190 |doi=10.1007/BF00290770 |last2=Rasa |first2=E.|bibcode=1983BEcoS..12..181A }}</ref>

===Resting and roosting===
<!--Roosting redirects here-->
{{Redirect|Roosting||Roost (disambiguation){{!}}Roost}}
[[File:Caribbean Flamingo2 (Phoenicopterus ruber) (0424) - Relic38.jpg|thumb|left|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. [[Avian sleep|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=Michel |last2=Tamisier |first2=Alain |last3=Cézilly |first3=Frank |title=Sleep-Vigilance Trade-Off in Gadwall During the Winter Period |journal=The Condor |date=2000 |volume=102 |issue=2 |pages=307 |doi=10.1650/0010-5422(2000)102[0307:SVTOIG]2.0.CO;2 |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|bibcode=2002JExpB.205..905B }}</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 }}</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 |doi=10.1007/BF00379484 |pmid=28310921 |bibcode=1985Oecol..68..126B |hdl=2027.42/47760 |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|journal=African Journal of Ecology |volume=56|issue=4|pages=882–884 |doi=10.1111/aje.12505 |doi-access=free|bibcode=2018AfJEc..56..882P }}</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'') |url=https://archive.org/details/sim_condor_1968-01_70_1/page/89 |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 }}</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 |volume=11 |issue=1 |pages=20018 |doi=10.1038/s41598-021-99296-8 |pmc=8501033 |pmid=34625581 |bibcode=2021NatSR..1120018C}}</ref>

===Breeding===
{{See also|Category:Avian sexuality|Animal sexual behaviour#Birds|Seabird breeding behaviour|Sexual selection in birds}}

====Social systems====
[[File:Raggiana Bird-of-Paradise wild 5.jpg|thumb|alt= Bird faces up with green face, black breast and pink lower body. Elaborate long feathers on the wings and tail.|right|Like others of its family, the male [[Raggiana bird-of-paradise]] has elaborate breeding plumage used to impress females.<ref>{{Cite journal|doi=10.1071/MU9810193|last=Frith|first=C. B.|title=Displays of Count Raggi's Bird-of-Paradise ''Paradisaea raggiana'' and congeneric species |journal=Emu|volume=81|issue=4|pages=193–201| url=http://www.publish.csiro.au/paper/MU9810193.htm|year=1981|bibcode=1981EmuAO..81..193F }}</ref>]]
95 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|bibcode=1987ANat..130..507F }}</ref> Monogamy allows for both [[paternal care]] and [[Parental investment|biparental care]], which is especially important for species in which care from both the female and the male parent is required in order to successfully rear a brood.<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|bibcode=1983ANat..121..149G }}</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. |last2=Buschhaus|first2=Nancy|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|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 }}</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>

Other mating systems, including [[Polygyny in animals|polygyny]], [[Polyandry in animals|polyandry]], [[Animal sexual behaviour#Polygamy|polygamy]], [[polygynandry]], and [[Promiscuity#Other animals|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 |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 |journal=Trends in Ecology & Evolution |language=en |volume=13 |issue=2 |pages=52–58 |doi=10.1016/S0169-5347(97)01232-9|pmid=21238200 |bibcode=1998TEcoE..13...52P }}</ref> A unique system is the formation of trios where a third individual is allowed by a breeding pair temporarily into the territory to assist with brood raising thereby leading to higher fitness.<ref>{{Cite journal |last1=Barve |first1=Sahas |last2=Riehl |first2=C. |last3=Walters |first3=E. L. |last4=Haydock |first4=J. |last5=Dugdale |first5=H. L. |last6=Koenig |first6=W. D. |date=2021 |title=Lifetime Reproductive Benefits of Cooperative Polygamy Vary for Males and Females in the Acorn Woodpecker (Melanerpes formicivorus). |journal=Proceedings of the Royal Society B: Biological Sciences |volume=208 |issue=1957 |pages=20210579|doi=10.1098/rspb.2021.0579 |pmid=34403633 |pmc=8370801 }}</ref><ref name=":0">{{Cite journal |last1=Roy |first1=Suhridam |last2=Kittur |first2=Swati |last3=Sundar |first3=K. S. Gopi |date=2022 |title=Sarus crane Antigone antigone trios and their triets: Discovery of a novel social unit in cranes |journal=Ecology |volume=103 |issue=6 |pages=e3707 |doi=10.1002/ecy.3707 |pmid=35357696 |bibcode=2022Ecol..103E3707R }}</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.|bibcode=2004BEcoS..57..110S }}</ref> [[Courtship feeding]], [[Billing (birds)|billing]] and {{Birdgloss|allopreening}} are commonly performed between partners, generally after the birds have paired and mated.<ref>{{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>

[[Homosexuality in animals#Birds|Homosexual behaviour has been observed]] in males or females in numerous species of birds, including copulation, pair-bonding, and joint parenting of chicks.<ref>{{cite book |last1=Bagemihl |first1=Bruce |title=Biological exuberance: Animal homosexuality and natural diversity |location=New York |publisher=St. Martin's |year=1999 |pages=479–655}}</ref> Over 130 avian species around the world engage in sexual interactions between the same sex or homosexual behaviours. "Same-sex courtship activities may involve elaborate displays, synchronised dances, gift-giving ceremonies, or behaviours at specific display areas including bowers, arenas, or leks."<ref>{{cite journal |last1=MacFarlane |first1=Geoff R. |last2=Blomberg |first2=Simon P. |last3=Kaplan |first3=Gisela |last4=Rogers |first4=Lesley J. |title=Same-sex sexual behavior in birds: expression is related to social mating system and state of development at hatching |journal=Behavioral Ecology |date=January 2007 |volume=18 |issue=1 |pages=21–33 |doi=10.1093/beheco/arl065 |hdl=10.1093/beheco/arl065 |hdl-access=free }}</ref>

====Territories, nesting and incubation====
{{See also|Bird nest}}
[[File:Bird-nest (2).jpg|alt=two unused bird nest|thumb|left|A bird nest which fell from a tree.]]
Many birds actively defend a territory from others of the same species during the breeding season; maintenance of territories protects the food source for their chicks. Species that are unable to defend feeding territories, such as [[seabird]]s and [[Swift (bird)|swift]]s, often breed in [[Bird colony|colonies]] instead; this is thought to offer protection from predators. Colonial breeders defend small nesting sites, and competition between and within species for nesting sites can be intense.<ref>{{cite journal |last1=Kokko |first1=H |last2=Harris |first2=M |last3=Wanless |first3=S |year=2004 |title=Competition for breeding sites and site-dependent population regulation in a highly colonial seabird, the common guillemot ''Uria aalge'' |journal=Journal of Animal Ecology |volume=73 |issue=2| pages=367–376 |doi=10.1111/j.0021-8790.2004.00813.x|doi-access=free |bibcode=2004JAnEc..73..367K }}</ref>

All birds lay [[amniotic egg]]s with hard shells made mostly of [[calcium carbonate]].<ref name = "Gill"/> Hole and burrow nesting species tend to lay white or pale eggs, while open nesters lay [[camouflage]]d eggs. There are many exceptions to this pattern, however; the ground-nesting [[nightjar]]s have pale eggs, and camouflage is instead provided by their plumage. Species that are victims of [[brood parasites]] have varying egg colours to improve the chances of spotting a parasite's egg, which forces female parasites to match their eggs to those of their hosts.<ref>{{cite journal | last1 = Booker | first1 = L | last2 = Booker | first2 = M | year = 1991 | title = Why Are Cuckoos Host Specific? | journal = [[Oikos (journal)|Oikos]] | volume = 57 | issue = 3| pages = 301–309 | doi = 10.2307/3565958 | jstor=3565958}}</ref>
[[File:Golden-backed Weaver.jpg|thumb|left|alt=Yellow weaver (bird) with black head hangs an upside-down nest woven out of grass fronds.|Male [[golden-backed weaver]]s construct elaborate suspended nests out of grass.]]
Bird eggs are usually laid in a [[Bird nest|nest]]. Most species create somewhat elaborate nests, which can be cups, domes, plates, mounds, or burrows.<ref name = "Hansell">{{cite book |last1=Hansell |first1=M |year=2000 |title=Bird Nests and Construction Behaviour |publisher=University of Cambridge Press |isbn=0-521-46038-7}}</ref> Some bird nests can be a simple scrape, with minimal or no lining; most seabird and wader nests are no more than a scrape on the ground. Most birds build nests in sheltered, hidden areas to avoid predation, but large or colonial birds—which are more capable of defence—may build more open nests. During nest construction, some species seek out plant matter from plants with parasite-reducing toxins to improve chick survival,<ref>{{cite journal | last1 = Lafuma | first1 = L. | last2 = Lambrechts | first2 = M. | last3 = Raymond | first3 = M. | year = 2001 | title = Aromatic plants in bird nests as a protection against blood-sucking flying insects? | journal = Behavioural Processes | volume = 56 | issue = 2| pages = 113–120 | doi = 10.1016/S0376-6357(01)00191-7 | pmid = 11672937 }}</ref> and feathers are often used for nest insulation.<ref name = "Hansell"/> Some bird species have no nests; the cliff-nesting [[common guillemot]] lays its eggs on bare rock, and male [[emperor penguin]]s keep eggs between their body and feet. The absence of nests is especially prevalent in open habitat ground-nesting species where any addition of nest material would make the nest more conspicuous. Many ground nesting birds lay a clutch of eggs that hatch synchronously, with [[precocial]] chicks led away from the nests ([[nidifugous]]) by their parents soon after hatching.<ref>{{cite book |author1=Collias, Nicholas E. |title=Nest building and bird behavior |author2=Collias, Elsie C. |publisher=Princeton University Press |year=1984 |isbn=0691083584 |place=Princeton, NJ |pages=16–17, 26}}</ref>

[[File:Eastern Phoebe-nest-Brown-headed-Cowbird-egg.jpg|thumb|alt= Nest made of straw with five white eggs and one grey speckled egg|Nest of an [[eastern phoebe]] that has been parasitised by a [[brown-headed cowbird]]]]
[[Egg incubation|Incubation]], which regulates temperature for chick development, usually begins after the last egg has been laid.<ref name = "Gill"/> In monogamous species incubation duties are often shared, whereas in polygamous species one parent is wholly responsible for incubation. Warmth from parents passes to the eggs through [[brood patch]]es, areas of bare skin on the abdomen or breast of the incubating birds. Incubation can be an energetically demanding process; adult albatrosses, for instance, lose as much as {{convert|83|g}} of body weight per day of incubation.<ref>{{cite book |last1=Warham |first1=J. |year=1990 |title=The Petrels: Their Ecology and Breeding Systems |location=London |publisher=[[Academic Press]] |isbn=0-12-735420-4}}</ref> The warmth for the incubation of the eggs of [[megapode]]s comes from the sun, decaying vegetation or volcanic sources.<ref>{{cite book |last1=Jones |first1=DN |last2=Dekker |first2=René WRJ |last3=Roselaar |first3=Cees S |year=1995 |chapter=The Megapodes |title=Bird Families of the World 3. |publisher=[[Oxford University Press]] |location=Oxford |isbn=0-19-854651-3}}</ref> Incubation periods range from 10 days (in [[woodpecker]]s, [[cuckoo]]s and [[passerine]] birds) to over 80 days (in albatrosses and [[Kiwi (bird)|kiwi]]s).<ref name = "Gill"/>

The diversity of characteristics of birds is great, sometimes even in closely related species. Several avian characteristics are compared in the table below.<ref name="AnAge">{{cite web|title=AnAge: The animal ageing and longevity database|url=http://genomics.senescence.info/species/|publisher=Human Ageing and Genomics Resources|access-date=26 September 2014}}</ref><ref name="ADW">{{cite web|title=Animal diversity web|url=http://animaldiversity.ummz.umich.edu/|publisher=University of Michigan, Museum of Zoology |access-date=26 September 2014}}</ref>

{| class="wikitable sortable"
|-
! Species
! Adult weight<br />(grams)
! Incubation<br />(days)
! Clutches<br />(per year)
! Clutch size
|-
| [[Ruby-throated hummingbird]] (''Archilochus colubris'')
| 3
| 13
| 2.0
| 2
|-
| [[House sparrow]] (''Passer domesticus'')
| 25
| 11
| 4.5
| 5
|-
| [[Greater roadrunner]] (''Geococcyx californianus'')
|  376
| 20
| 1.5
| 4
|-
| [[Turkey vulture]] (''Cathartes aura'')
| 2,200
| 39
| 1.0
| 2
|-
| [[Laysan albatross]] (''Phoebastria immutabilis'')
| 3,150
| 64
| 1.0
| 1
|-
| [[Magellanic penguin]] (''Spheniscus magellanicus'')
| 4,000
| 40
| 1.0
| 1
|-
| [[Golden eagle]] (''Aquila chrysaetos'')
| 4,800
| 40
| 1.0
| 2
|-
| [[Wild turkey]] (''Meleagris gallopavo'')
| 6,050
| 28
| 1.0
| 11
|}

====Parental care and fledging====
{{Main|Parental care in birds}}
At the time of their hatching, chicks range in development from helpless to independent, depending on their species. Helpless chicks are termed ''[[altricial]]'', and tend to be born small, [[Blindness|blind]], immobile and naked; chicks that are mobile and feathered upon hatching are termed ''[[precocial]]''. Altricial chicks need help [[thermoregulation|thermoregulating]] and must be brooded for longer than precocial chicks. The young of many bird species do not precisely fit into either the precocial or altricial category, having some aspects of each and thus fall somewhere on an "altricial-precocial spectrum".<ref name="Urfi2011">{{cite book|last=Urfi|first=A. J.|title=The Painted Stork: Ecology and Conservation|url=https://books.google.com/books?id=_9tczTapYXMC&pg=PA88|year=2011|publisher=Springer Science & Business Media|isbn=978-1-4419-8468-5|page=88}}</ref> Chicks at neither extreme but favouring one or the other may be termed {{Birdgloss|semi-precocial}}<ref name="Khanna2005">{{cite book|last=Khanna|first=D. R.|title=Biology of Birds|url=https://books.google.com/books?id=fDblIChi7KwC&pg=PA109|year=2005|publisher=Discovery Publishing House|isbn=978-81-7141-933-3|page=109}}</ref> or {{Birdgloss|semi-altricial}}.<ref name="Scott2008">{{cite book|last=Scott|first=Lynnette|title=Wildlife Rehabilitation|url=https://books.google.com/books?id=FpAOAQAAMAAJ|year=2008|publisher=National Wildlife Rehabilitators Association|isbn=978-1-931439-23-7|page=50}}</ref>
[[File:White-breasted Woodswallow chicks in nest.jpg|thumb|alt=Looking down on three helpless blind chicks in a nest within the hollow of a dead tree trunk|right|[[Altricial]] chicks of a [[white-breasted woodswallow]]]]

The length and nature of parental care varies widely amongst different orders and species. At one extreme, parental care in [[megapode]]s ends at hatching; the newly hatched chick digs itself out of the nest mound without parental assistance and can fend for itself immediately.<ref>{{cite book |last1=Elliot |first1=A |year=1994 |chapter=Family Megapodiidae (Megapodes) |title=Handbook of the Birds of the World |series=Vol. 2: New World Vultures to Guineafowl |editor-last1=del Hoyo |editor-first1=J. |editor-last2=Elliott |editor-first2=A. |editor-last3=Sargatal |editor-first3=J. |publisher=Lynx Edicions |location=Barcelona |isbn=84-87334-15-6 |title-link=Handbook of the Birds of the World }}</ref> At the other extreme, many seabirds have extended periods of parental care, the longest being that of the [[great frigatebird]], whose chicks take up to six months to [[fledge]] and are fed by the parents for up to an additional 14 months.<ref>{{cite book |last1=Metz |first1=V. G. |last2=Schreiber |first2=E. A. |year=2002|chapter=Great Frigatebird (''Fregata minor'') |title=The Birds of North America, No 681 |editor-last1=Poole |editor-first1=A. |editor-last2=Gill |editor-first2=F. |publisher=The Birds of North America Inc |location=Philadelphia}}</ref> The ''chick guard stage'' describes the period of breeding during which one of the adult birds is permanently present at the nest after chicks have hatched. The main purpose of the guard stage is to aid offspring to thermoregulate and protect them from predation.<ref>{{cite book |last1=Young |first1=Euan |title=Skua and Penguin. Predator and Prey |publisher=Cambridge University Press |year=1994 |page=453}}</ref>
[[File:Calliope-nest edit.jpg|thumb|alt=Hummingbird perched on edge of tiny nest places food into mouth of one of two chicks|left|A female [[calliope hummingbird]] feeding fully grown chicks]]
In some species, both parents care for nestlings and fledglings; in others, such care is the responsibility of only one sex. In some species, [[helpers at the nest|other members]] of the same species—usually close relatives of the [[breeding pair]], such as offspring from previous broods—will help with the raising of the young.<ref>{{cite journal | last1 = Ekman | first1 = J. | year = 2006 | title = Family living amongst birds | journal = [[Journal of Avian Biology]] | volume = 37 | issue = 4| pages = 289–298 | doi = 10.1111/j.2006.0908-8857.03666.x }}</ref> Such [[alloparenting]] is particularly common among the [[Corvida]], which includes such birds as the true [[Corvidae|crows]], [[Australian magpie]] and [[fairy-wren]]s,<ref>{{Cite book|vauthors=Cockburn A |veditors=Floyd R, Sheppard A, de Barro P |title=Frontiers in Population Ecology|year=1996|publisher=CSIRO|location=Melbourne|pages=21–42|chapter=Why do so many Australian birds cooperate? Social evolution in the Corvida}}</ref> but has been observed in species as different as the [[Rifleman (bird)|rifleman]] and [[red kite]]. Among most groups of animals, [[Paternal care|male parental care]] is rare. In birds, however, it is quite common—more so than in any other vertebrate class.<ref name = "Gill"/> Although territory and nest site defence, incubation, and chick feeding are often shared tasks, there is sometimes a [[division of labour]] in which one mate undertakes all or most of a particular duty.<ref>{{Cite journal|last=Cockburn|first=Andrew|year=2006|title=Prevalence of different modes of parental care in birds |doi=10.1098/rspb.2005.3458|journal=[[Proceedings of the Royal Society B]]|volume=273|issue=1592|pages=1375–1383|pmid=16777726|pmc=1560291}}</ref>

The point at which chicks [[fledge]] varies dramatically. The chicks of the ''[[Synthliboramphus]]'' murrelets, like the [[ancient murrelet]], leave the nest the night after they hatch, following their parents out to sea, where they are raised away from terrestrial predators.<ref>{{cite book |last1=Gaston |first1=AJ |year=1994 |chapter=Ancient Murrelet (''Synthliboramphus antiquus'') |title=The Birds of North America, No. 132 |editor-first1=A. |editor-last1=Poole |editor-first2=F. |editor-last2=Gill |location=Philadelphia & Washington, D.C. |publisher=The Academy of Natural Sciences & The American Ornithologists' Union}}</ref> Some other species, such as ducks, move their chicks away from the nest at an early age. In most species, chicks leave the nest just before, or soon after, they are able to fly. The amount of parental care after fledging varies; albatross chicks leave the nest on their own and receive no further help, while other species continue some supplementary feeding after fledging.<ref>{{cite journal | last1 = Schaefer | first1 = H. C. | last2 = Eshiamwata | first2 = G. W. | last3 = Munyekenye | first3 = F. B. | last4 = Böhning-Gaese | first4 = K. | year = 2004 | title = Life-history of two African ''Sylvia'' warblers: low annual fecundity and long post-fledging care | journal = [[Ibis (journal)|Ibis]] | volume = 146 | issue = 3| pages = 427–437 | doi = 10.1111/j.1474-919X.2004.00276.x }}</ref> Chicks may also follow their parents during their first [[bird migration|migration]].<ref>{{cite journal | last1 = Alonso | first1 = J. C. | last2 = Bautista | first2 = L. M. | last3 = Alonso | first3 = J. A. | year = 2004 | title = Family-based territoriality vs flocking in wintering common cranes ''Grus grus'' | journal = [[Journal of Avian Biology]] | volume = 35 | issue = 5| pages = 434–444 | doi = 10.1111/j.0908-8857.2004.03290.x | hdl = 10261/43767 }}</ref>

====Brood parasites====
{{Main|Brood parasite}}
[[File:Reed warbler cuckoo.jpg|thumb|upright|right|alt=Small brown bird places an insect in the bill of much larger grey bird in nest|[[Reed warbler]] raising a [[common cuckoo]], a [[brood parasite]]]]
[[Brood parasitism]], in which an egg-layer leaves her eggs with another individual's brood, is more common among birds than any other type of organism.<ref name="brood">{{cite book |last1=Davies |first1=N. |year=2000 |title=Cuckoos, Cowbirds and other Cheats |publisher=[[T. & A. D. Poyser]] |location=London |isbn=0-85661-135-2}}</ref> After a parasitic bird lays her eggs in another bird's nest, they are often accepted and raised by the host at the expense of the host's own brood. Brood parasites may be either ''obligate brood parasites'', which must lay their eggs in the nests of other species because they are incapable of raising their own young, or ''non-obligate brood parasites'', which sometimes lay eggs in the nests of [[conspecific]]s to increase their reproductive output even though they could have raised their own young.<ref>{{cite journal |doi=10.1093/beheco/8.2.153 |last1=Sorenson |first1=M. |year=1997 |title=Effects of intra- and interspecific brood parasitism on a precocial host, the canvasback, ''Aythya valisineria'' |journal=Behavioral Ecology |volume=8 |issue=2| pages=153–161 |doi-access=free}}</ref> One hundred bird species, including [[honeyguide]]s, [[icterid]]s, and [[Black-headed duck|ducks]], are obligate parasites, though the most famous are the [[cuckoo]]s.<ref name="brood"/> Some brood parasites are adapted to hatch before their host's young, which allows them to destroy the host's eggs by pushing them out of the nest or to kill the host's chicks; this ensures that all food brought to the nest will be fed to the parasitic chicks.<ref>{{cite journal| last1=Spottiswoode| first1=C. N.| last2=Colebrook-Robjent| first2=J. F. R.| title=Egg puncturing by the brood parasitic Greater Honeyguide and potential host counteradaptations| journal=Behavioral Ecology| volume=18| pages=792–799| year=2007| doi=10.1093/beheco/arm025| issue=4| doi-access=free| hdl=10.1093/beheco/arm025| hdl-access=free}}</ref>

====Sexual selection====
[[File:Peacock Flying.jpg|thumb|upright=1.35|right|The peacock tail in flight, the classic example of a [[Fisherian runaway]]]]
{{Main|Sexual selection in birds}}
Birds have [[evolution|evolved]] a variety of [[mating]] behaviours, with the [[peafowl|peacock]] tail being perhaps the most famous example of [[sexual selection]] and the [[Fisherian runaway]]. Commonly occurring [[sexual dimorphism]]s such as size and colour differences are energetically costly attributes that signal competitive breeding situations.<ref name=edwards2012>{{cite journal|last=Edwards|first=DB|title=Immune investment is explained by sexual selection and pace-of-life, but not longevity in parrots (Psittaciformes).|journal=PLOS ONE|year=2012|volume=7|issue=12|pages=e53066|pmid=23300862|doi=10.1371/journal.pone.0053066|pmc=3531452|bibcode=2012PLoSO...753066E|doi-access=free}}</ref> Many types of avian [[sexual selection]] have been identified; intersexual selection, also known as female choice; and intrasexual competition, where individuals of the more abundant sex compete with each other for the privilege to mate. Sexually selected traits often evolve to become more pronounced in competitive breeding situations until the trait begins to limit the individual's fitness. Conflicts between an individual fitness and signalling adaptations ensure that sexually selected ornaments such as plumage colouration and [[courtship behavior|courtship behaviour]] are "honest" traits. Signals must be costly to ensure that only good-quality individuals can present these exaggerated sexual ornaments and behaviours.<ref name=doutrelant2012>{{cite journal|last=Doutrelant|first=C|author2=Grégoire, A |author3=Midamegbe, A |author4=Lambrechts, M |author5= Perret, P |title=Female plumage coloration is sensitive to the cost of reproduction. An experiment in blue tits.|journal=[[Journal of Animal Ecology]]|date=January 2012|volume=81|issue=1|pages=87–96|pmid=21819397|doi=10.1111/j.1365-2656.2011.01889.x|doi-access=free|bibcode=2012JAnEc..81...87D}}</ref>

====Inbreeding depression====
{{Main|Inbreeding depression}}
Inbreeding causes early death ([[inbreeding depression]]) in the [[zebra finch]] ''Taeniopygia guttata''.<ref name="pmid22643890">{{cite journal |vauthors=Hemmings NL, Slate J, Birkhead TR |title=Inbreeding causes early death in a passerine bird |journal=Nat Commun |volume=3 |page=863 |year=2012 |pmid=22643890 |doi=10.1038/ncomms1870 |bibcode=2012NatCo...3..863H |doi-access=free }}</ref> Embryo survival (that is, hatching success of fertile eggs) was significantly lower for [[sibling|sib-sib]] mating pairs than for unrelated pairs.<ref>{{Cite journal |last1=Hemmings |first1=N. L. |last2=Slate |first2=J. |last3=Birkhead |first3=T. R. |date=29 May 2012 |title=Inbreeding causes early death in a passerine bird |journal=Nature Communications |language=en |volume=3 |issue=1 |pages=863 |doi=10.1038/ncomms1870 |pmid=22643890 |bibcode=2012NatCo...3..863H |doi-access=free }}</ref>

[[Darwin's finches|Darwin's finch]] ''Geospiza scandens'' experiences [[inbreeding depression]] (reduced survival of offspring) and the magnitude of this effect is influenced by environmental conditions such as low food availability.<ref name="pmid12144022">{{cite journal |vauthors=Keller LF, Grant PR, Grant BR, Petren K |title=Environmental conditions affect the magnitude of inbreeding depression in survival of Darwin's finches |journal=Evolution |volume=56 |issue=6 |pages=1229–1239 |year=2002 |pmid=12144022 |doi=10.1111/j.0014-3820.2002.tb01434.x }}</ref>

====Inbreeding avoidance====
{{Main|Inbreeding avoidance}}
Incestuous matings by the [[Purple-crowned fairywren|purple-crowned fairy wren]] ''Malurus coronatus'' result in severe fitness costs due to [[inbreeding depression]] (greater than 30% reduction in hatchability of eggs).<ref name=Kingma>{{cite journal | last1 = Kingma | first1 = SA | last2 = Hall | first2 = ML | last3 = Peters | first3 = A | year = 2013 | title = Breeding synchronization facilitates extrapair mating for inbreeding avoidance | journal = Behavioral Ecology | volume = 24 | issue = 6| pages = 1390–1397 | doi = 10.1093/beheco/art078 | doi-access = free | hdl = 10.1093/beheco/art078 | hdl-access = free }}</ref> Females paired with related males may undertake extra pair matings (see [[Promiscuity#Other animals]] for 90% frequency in avian species) that can reduce the negative effects of inbreeding. However, there are ecological and demographic constraints on extra pair matings. Nevertheless, 43% of broods produced by incestuously paired females contained extra pair young.<ref name=Kingma />

Inbreeding depression occurs in the [[great tit]] (''Parus major'') when the offspring produced as a result of a mating between close relatives show reduced fitness. In natural populations of ''Parus major'', inbreeding is avoided by dispersal of individuals from their birthplace, which reduces the chance of mating with a close relative.<ref name="pmid18211876">{{cite journal |vauthors=Szulkin M, Sheldon BC |title=Dispersal as a means of inbreeding avoidance in a wild bird population |journal=Proc. Biol. Sci. |volume=275 |issue=1635 |pages=703–711 |year=2008 |pmid=18211876 |pmc=2596843 |doi=10.1098/rspb.2007.0989 }}</ref>

[[Southern pied babbler]]s ''Turdoides bicolor'' appear to avoid inbreeding in two ways. The first is through dispersal, and the second is by avoiding familiar group members as mates.<ref name="pmid22471769">{{cite journal |vauthors=Nelson-Flower MJ, Hockey PA, O'Ryan C, Ridley AR |title=Inbreeding avoidance mechanisms: dispersal dynamics in cooperatively breeding southern pied babblers |journal=J Anim Ecol |volume=81 |issue=4 |pages=876–883 |year=2012 |pmid=22471769 |doi=10.1111/j.1365-2656.2012.01983.x |doi-access=free |bibcode=2012JAnEc..81..876N }}</ref>

[[Cooperative breeding]] in birds typically occurs when offspring, usually males, delay dispersal from their natal group in order to remain with the family to help rear younger kin.<ref name="pmid26577076">{{cite journal |vauthors=Riehl C, Stern CA |title=How cooperatively breeding birds identify relatives and avoid incest: New insights into dispersal and kin recognition |journal=BioEssays |volume=37 |issue=12 |pages=1303–1308 |year=2015 |pmid=26577076 |doi=10.1002/bies.201500120 }}</ref> Female offspring rarely stay at home, dispersing over distances that allow them to breed independently, or to join unrelated groups. In general, inbreeding is avoided because it leads to a reduction in progeny fitness ([[inbreeding depression]]) due largely to the homozygous expression of deleterious recessive alleles.<ref name="pmid19834483">{{cite journal |vauthors=Charlesworth D, Willis JH |title=The genetics of inbreeding depression |journal=Nat. Rev. Genet. |volume=10 |issue=11 |pages=783–796 |year=2009 |pmid=19834483 |doi=10.1038/nrg2664 }}</ref> [[Outcrossing|Cross-fertilisation]] between unrelated individuals ordinarily leads to the masking of deleterious recessive alleles in progeny.<ref name="pmid3324702">{{cite book |vauthors=Bernstein H, Hopf FA, Michod RE |title=Molecular Genetics of Development |chapter=The Molecular Basis of the Evolution of Sex |volume=24 |pages=323–370 |year=1987 |pmid=3324702 |doi= 10.1016/s0065-2660(08)60012-7|series=Advances in Genetics |isbn=9780120176243 }}</ref><ref>{{cite book |last1=Michod |first1=R.E. |year=1994 |title=Eros and Evolution: A Natural Philosophy of Sex |publisher=Addison-Wesley Publishing Company |location=Reading, Massachusetts |isbn=978-0201442328}}</ref>