The breeding behaviour of birds encompasses a range of activities associated with the reproductive cycle of birds, from courtship to the fledging of the young. This behaviour includes mate selection, nest building, egg laying, incubation, and parental care until the young become fledged. During this period, birds actively defend their territory. Most species provide extensive care to their offspring, though exceptions exist. Approximately 90% of bird species are monogamous during a breeding season, with females typically choosing mates based on appearance, skills, and song. In polygamous species, such as pheasants, males often do not participate in raising young, leaving the responsibility to females.

Breeding site Number of individuals at nesting sites

The number of birds at a nesting site largely depends on food availability. Seabirds typically nest in large colonies due to limited suitable locations, which are often far apart. These birds return to the same site annually, usually breeding with the same partner and reusing the same nesting spot for years.

Terrestrial birds face fewer constraints and may select new territories each year. However, cavity nesters, such as woodpeckers, are limited by the availability of suitable tree cavities. A cavity must have the right diameter, and even a brief absence can result in it being occupied by another bird.

Nesting season The timing of breeding varies by region. In northern Europe, nesting is influenced by day length and temperature, with chicks hatching when food is most abundant. Equatorial species may breed year-round. Nomadic desert birds, such as budgerigar and zebra finch, begin breeding at the onset of the rainy season. Similarly, the central Australian desert lark typically nests between July and November but will breed opportunistically if rains occur outside this period. The breeding of Snow petrel depends on snow accumulation, nesting up to 300 km inland in Antarctica. Frozen nesting sites prevent breeding.

Territory

Males initiate the breeding season by claiming a territory sufficient to provide food for their young, though this varies by territory type. Territories range from 90 cm² to several kilometres in diameter, depending on the species. Pair formation occurs within the territory, and nests are often, but not always, built there.

Males aggressively defend their territory, primarily through song to signal occupancy. Physical fights are rare and seldom result in injury or death. An exception is the European robin, which fiercely defends its territory, sometimes leading to severe injury or death.

Common murres have the smallest territories, laying a single egg on a cliff ledge with just a few centimetres of space. Despite their 42 cm body length, pairs can nest at a density of 20 per m².

Habitat selection Not all habitats are equally suitable for nesting. Evolutionarily, habitats that support higher offspring survival enhance fitness. Birds initially target optimal sites, but high density can reduce habitat quality, making it comparable to less desirable sites. Poorer habitats support fewer pairs, leading to higher nest density in better habitats, balancing overall quality. For example, in an American herring gull colony on Newfoundland's Great Island, rocky ledges (10% of the area but hosting 22% of the population) are preferred, with gulls laying more eggs and chicks growing faster than in meadows or on grassy slopes. However, high density increases chick mortality, resulting in similar breeding success across habitats (about half the fledglings per egg laid).

Habitat modification Materials brought to nesting colonies and accumulated guano contribute to soil formation in some conditions, creating ornithogenic soils.

Reproductive strategies

About 90% of birds are monogamous within a breeding season, maintaining pair bonds during breeding and often raising subsequent broods together. Some species, like white storks and mute swans, form lifelong pairs. However, white storks are more attached to their nest than their partner. Males arrive days earlier to repair the nest, and if the female does not return from Africa, another female may take her place, forming a new pair.

Polygamous birds, such as galliformes, have multiple partners. Males aim to mate with as many females as possible in a season. Some monogamous birds engage in extra-pair copulations while maintaining their primary partner, known as social monogamy.

Other strategies exist, such as promiscuity in the aquatic warbler, where multiple males and females mate interchangeably, resulting in clutches with offspring from up to five males.

Male common blackbirds may collectively force copulation on females. Male mallards, despite being monogamous, engage in similar behaviour, sometimes leading to the female being drowned. This can result in interspecies hybrids, even outside the genus Anas.

Female mimicry In the ruff, a unique form of female mimicry occurs, with some males, called faeders, resembling females. While typical males are larger and more brightly coloured with strong sexual dimorphism, faeders are only slightly larger than females, adopt female-like plumage during the breeding season, but possess testicles 2.5 times larger than those of typical males. Faeders, comprising around 1% of the population, form a cooperative relationship with dominant males. Copulations involve various combinations, including dominant males mating with faeders or females, and faeders mating with other males or females. Initially thought to represent an ancestral behaviour where males resembled females for shared parental care (faeder meaning "father" in Old English), current evidence suggests faeders exploit opportunities to fertilise females paired with other males.

Mate identification Mate identification in many birds involves imprinting. Newly hatched chicks instinctively focus on a large, moving object, which becomes the template for their parent during the chick stage and their mate during adulthood. In natural conditions, this is almost always the actual parent, leading adults to seek mates of their own species. However, if an egg or chick is transferred to another nest or incubator, such as in farms or zoos, the chick may imprint on a different species. As adults, these birds display courtship behaviour toward that other species, using gestures typical of their own species, which often fails to convey intent. Common cases involve humans (breeders), but examples include geese courting chickens or peafowl courting turtles. Such behaviour has also been observed in house sparrows and western jackdaws.

This imprinting also affects captive birds. Society finches incubate foreign eggs and raise chicks as their own, but these chicks, as adults, prefer to mate with society finches rather than their own species. Similarly, European goldfinches may court their human caretakers.

Courtship

Male displays

Females typically choose males based on appearance, skills, and song. In polygamous species like pheasants, males often do not contribute to chick-rearing, leaving it to females.

Males may display vibrant plumage or sing to attract mates, often adopting a breeding plumage. Male rock sparrows puff up their breast feathers to reveal a normally hidden yellow breast patch. Studies on great tits show females prefer males with wider black belly stripes, confirmed by experiments where thicker stripes were artificially added, making males more attractive and perceived as dominant.

In song-based courtship, the loudest or most complex singer is often chosen. Singing also signals territory occupancy, with males perching high to project sound.

In some species, like the Eurasian dotterel, females court males, displaying brighter plumage. Males then handle incubation, nest maintenance, and chick care.

Lekking grounds Lekking grounds are sites where males gather to display, typically at dawn or occasionally dusk, aiming to position themselves near the centre. Common pheasants lek alone on small patches of trampled grass, emitting hoarse calls every few minutes, while species like manakins, cotingas, and hummingbirds display on branches or in flight.

Display flights may showcase agility, often accompanied by song. For example, Eurasian skylark and common snipe perform aerial displays, with the snipe's tail feathers vibrating to produce a buzzing sound.

Feather preening and feeding

Birds preen feathers to clean and align them, but this also strengthens pair bonds, continuing long after pairing. Preening can reduce aggression when another bird approaches too closely, as seen in parrots and estrildid finches, which appear to enjoy physical contact.

Feeding a partner, observed in common kingfishers, demonstrates a male's ability to provide for offspring. Males offer fish, sometimes repeatedly, to prove their capability. Gulls exhibit similar behaviour, with females emitting begging calls resembling the calls of young chicks. Male Eurasian blue tits feed females to build energy reserves for egg-laying and to reinforce pair bonds. Birds may also offer nesting materials.

Mating dances

Species like great crested grebes and cranes perform species-specific mating dances to strengthen pair bonds. Subtle differences in dances prevent hybridisation between closely related species. Errors in dance movements can lead to pair dissolution, typically after a second mistake. Dances range from seconds to hours.

Bald eagles perform a unique aerial dance, where the female flips upside-down in mid-flight, and the male grasps her talons.

Bowers Male bowerbirds construct bowers, structures composed of upright grass and sticks, decorated with feathers, fruit, stones, or debris. Depending on the species, the bowers resemble tunnels or thatched huts, up to 1.5 metres tall. Satin bowerbirds prefer blue decorations. Females assess the bower quality, and if impressed, mate with the male. The female then builds a separate nest for egg-laying.

Copulation

Copulation typically lasts 1–2 seconds but can be longer in some species, such as the aquatic warbler, where captive observations recorded 40-minute sessions. Most birds lack a penis, requiring cloaca contact. The male mounts the female, balancing with gentle wing flaps.

Nest Most birds build nests, though emperor penguins and king penguins warm eggs on their feet and beneath their brood patch, and gannets incubate their eggs between their webbed feet. Passerines usually abandon nests after breeding, while Eurasian magpie and hooded crow nests may be reused by long-eared owls and common kestrels. Bird of prey and those with large nests, like storks, reinforce nests annually, with older, greyer layers at the bottom and fresher, browner layers on top. Building a nest with thousands of pieces of moss, grass, and hair takes several days.

Nests are often lined with soft materials like feathers, hair, or fine roots. Urban birds, including storks, may incorporate debris, posing risks like water accumulation in plastic-lined nests, drowning chicks, or entanglement in string causing strangulation or necrosis.

Nests are typically placed in inaccessible locations. Grebes build floating nests, relatively safe from predators; they cover eggs with weed when leaving. Burrowing is rare, but is seen in species like common shelducks, puffins, and many petrels. Passerines nest in trees, grass, or shrubs, while raptors nest in trees, except for the hen harrier, which nests in tall ground vegetation. Western house martins initially build nests with a small entrance, widened after completion.

Nest-building duties vary by species. Females often lead, with males assisting in some cases. Male masked weavers build grass nests alone on branch tips. Female Eurasian chaffinches build with male supervision. Female pheasants handle all nesting tasks unaided.