Most birds have monogamous mating patterns, meaning that one male and one female mate exclusively with each other for at least one season. However, some bird species are either polygynous, that is, the males mate with more than one female, or polyandrous, in which case the females mate with more than one male (see Animal Courtship and Mating). Among many types of birds, including some jays, several adults, rather than a single breeding pair, often help to raise the young within an individual nest.

Birds rely heavily on their two main senses, vision and hearing, in courtship and breeding. Among most songbirds, including the nightingale and the sky lark, males use song to establish breeding territories and attract mates. In many species, female songbirds may be attracted to males that sing the loudest, longest, or most varied songs. Many birds, including starlings, mimic the sounds of other birds. This may help males to achieve sufficiently varied songs to attract females. Numerous birds rely on visual displays of their feathers to obtain a mating partner. For example, the blue bird of paradise hangs upside down from a tree branch to show off the dazzling feathers of its body and tail. A remarkable courtship strategy is exhibited by male bowerbirds of Australia and New Guinea. These birds attract females by building bowers for shelter, which they decorate with colorful objects such as flower petals, feathers, fruit, and even human-made items such as ribbons and tinfoil.

very large bird cages

Many birds establish breeding territories, which they defend from rivals of the same species. In areas where suitable nesting habitat is limited, birds may nest in large colonies. An example is the crab plover, which sometimes congregates by the thoUKnds in areas of only about 0.6 hectares (about 1.5 acres).

For breeding, most birds build nests, which help them to incubate, or warm, the developing eggs. Nests sometimes offer camouflage from predators and physical protection from the elements. Nests may be elaborate constructions or a mere scrape on the ground. Some birds, including many shorebirds, incubate their eggs without any type of nest at all. The male emperor penguin of icy Antarctica incubates the single egg on top of its feet under a fold of skin.

Bird nests range in size from the tiny cups of hummingbirds to the huge stick nests of eagles, which may weigh a ton or more. Some birds, such as the malleefowl of southern Australia, use external heat sources, such as decaying plant material, to incubate their eggs. Many birds, including woodpeckers, use tree cavities for nests. Others, such as cowbirds and cuckoos, are brood parasites; they neither build nests nor care for their young. Instead, females of these species lay their eggs in the nests of birds of other species, so that the eggs are incubated-and the hatchlings raised-by birds other than the hatchlings' true parents.

Incubation by one or both parents works together with the nest structure to provide an ideal environment for the eggs. The attending parent may warm the eggs with a part of its belly called the brood patch. Bird parents may also wet or shade the eggs to prevent them from overheating.

The size, shape, color, and texture of a bird egg is specific to each species. Eggs provide an ideal environment for the developing embryo. The shells of eggs are made from calcium carbonate. They contain thoUKnds of pores through which water can evaporate and air can seep in, enabling the developing embryo to breathe. The number of eggs in a clutch (the egg or eggs laid by a female bird in one nesting effort) may be 15 or more for some birds, including pheasants. In contrast, some large birds, such as condors and albatross, may lay only a single egg every two years. The eggs of many songbirds hatch after developing for as few as ten days, whereas those of albatross and kiwis may require 80 days or more.

Among some birds, including songbirds and pelicans, newly hatched young are without feathers, blind, and incapable of regulating their body temperature. Many other birds, such as ducks, are born covered with down and can feed themselves within hours after hatching. Depending on the species, young birds may remain in the nest for as little as part of a day or as long as several months. Fledged young (those that have left the nest) may still rely on parental care for many days or weeks. Only about 10 percent of birds survive their first year of life; the rest die from starvation, disease, predators, or inexperience with the behaviors necessary for survival. The age at which birds begin to breed varies from less than a year in many songbirds and some quail to ten years or more in some albatross. The life spans of birds in the wild are poorly known. Many small songbirds live only three to five years, whereas some albatross are known to have survived more than 60 years in the wild.