Editing Bird (section)

===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>