Bird nest

A bird nest is the spot in which a bird lays and incubates its eggs and raises its young. Although the term popularly refers to a specific structure made by the bird itself—such as the grassy cup nest of the American robin or Eurasian blackbird, or the elaborately woven hanging nest of the Montezuma oropendola or the village weaver—that is too restrictive a definition. For some species, a nest is simply a shallow depression made in sand; for others, it is the knot-hole left by a broken branch, a burrow dug into the ground, a chamber drilled into a tree, an enormous rotting pile of vegetation and earth, a shelf made of dried saliva or a mud dome with an entrance tunnel. The smallest bird nests are those of some hummingbirds, tiny cups which can be a mere 2 cm (0.79 in) across and 2–3 cm (0.79–1.18 in) high. At the other extreme, some nest mounds built by the dusky scrubfowl measure more than 11 m (36 ft) in diameter and stand nearly 5 m (16 ft) tall.

Not all bird species build nests. Some species lay their eggs directly on the ground or rocky ledges, while brood parasites lay theirs in the nests of other birds, letting unwitting "foster parents" do the work of rearing the young. Although nests are primarily used for breeding, they may also be reused in the non-breeding season for roosting and some species build special dormitory nests or roost nests (or winter-nest) that are used only for roosting.Most birds build a new nest each year, though some refurbish their old nests.The large eyries (or aeries) of some eagles are platform nests that have been used and refurbished for several years.

In most species, the female does most or all of the nest construction, though the male often helps.In some polygynous species, however, the male does most or all of the nest building. The nest may also form a part of the courtship display such as in weaver birds. The ability to choose and maintain good nest sites and build high quality nests may be selected for by females in these species. In some species the young from previous broods may also act as helpers for the adults.

Types 
Not every bird species builds or uses a nest. Some auks, for instance—including common murre, thick-billed murre and razorbill—lay their eggs directly onto the narrow rocky ledges they use as breeding sites.The eggs of these species are dramatically pointed at one end, so that they roll in a circle when disturbed. This is critical for the survival of the developing eggs, as there are no nests to keep them from rolling off the side of the cliff. Presumably because of the vulnerability of their unprotected eggs, parent birds of these auk species rarely leave them unattended. Nest location and architecture is strongly influenced by local topography and other abiotic factors. 

King penguins and emperor penguins also do not build nests; instead, they tuck their eggs and chicks between their feet and folds of skin on their lower bellies. They are thus able to move about while incubating, though in practice only the emperor penguin regularly does so. Emperor penguins breed during the harshest months of the Antarctic winter, and their mobility allows them to form huge huddled masses which help them to withstand the extremely high winds and low temperatures of the season. Without the ability to share body heat (temperatures in the centre of tight groups can be as much as 10C above the ambient air temperature), the penguins would expend far more energy trying to stay warm, and breeding attempts would probably fail. 

Some crevice-nesting species, including ashy storm-petrel, pigeon guillemot, Eurasian eagle-owl and Hume's tawny owl, lay their eggs in the relative shelter of a crevice in the rocks or a gap between boulders, but provide no additional nest material. Potoos lay their single egg directly atop a broken stump, or into a shallow depression on a branch—typically where an upward-pointing branch died and fell off, leaving a small scar or knot-hole. Brood parasites, such as the New World cowbirds, the honeyguides, and many of the Old World and Australasian cuckoos, lay their eggs in the active nests of other species.

Nest protection and sanitation 
Many species of bird conceal their nests to protect them from predators. Some species may choose nest sites that are inaccessible or build the nest so as to deter predators.Bird nests can also act as habitats for other inquiline species which may not affect the bird directly. Birds have also evolved nest sanitation measures to reduce the effects of parasites and pathogens on nestlings.

Some aquatic species such as grebes are very careful when approaching and leaving the nest so as not to reveal the location. Some species will use leaves to cover up the nest prior to leaving.

Ground birds such as plovers may use broken wing or rodent run displays to distract predators from nests. 

Many species attack predators or apparent predators near their nests. Kingbirds attack other birds that come too close. In North America, northern mockingbirds, blue jays, and Arctic terns can peck hard enough to draw blood.In Australia, a bird attacking a person near its nest is said to swoop the person. The Australian magpie is particularly well known for this behavior. 

Nests can become home to many other organisms including parasites and pathogens.The excreta of the fledglings also pose a problem. In most passerines, the adults actively dispose the fecal sacs of young at a distance or consume them. This is believed to help prevent ground predators from detecting nests.Young birds of prey however usually void their excreta beyond the rims of their nests. Blowflies of the genus Protocalliphora have specialized to become obligate nest parasites with the maggots feeding on the blood of nestlings.

Some birds have been shown to choose aromatic green plant material for constructing nests that may have insecticidal properties,while others may use materials such as carnivore scat to repel smaller predators.Some urban birds, house sparrows and house finches in Mexico, have been adopted the use of cellulose from cigarette butts which may be adaptive since these fibres contain nicotine and other toxic substances that repel ectoparasites. 

Some birds use pieces of snake slough in their nests.It has been suggested that these may deter some nest predators such as squirrels. 

Colonial nesting 
Though most birds nest individually, some species—including seabirds, penguins, flamingos, many herons, gulls, terns, weaver, some corvids and some sparrows—gather together in sizeable colonies. Birds that nest colonially may benefit from increased protection against predation. They may also be able to better utilize food supplies, by following more successful foragers to their foraging sites. 

In human culture 
Many birds nest close to human habitations and some have been specially encouraged. Nesting white storks have been protected and held in reverence in many cultures. Nest boxes are often used to encourage cavity nesting birds. The nesting of peregrine falcons on tall buildings has captured popular interest.Colonial breeders produce guano which is a valuable fertilizer. The saliva nest of the edible-nest swiftlet is used to make bird's nest soup,long considered a delicacy in China.Collection of the swiftlet nests is big business: in one year, more than 3.5 million nests were exported from Borneo to China,and the industry was estimated at $1 billion US per year (and increasing) in 2008.While the collection is regulated in some areas (at the Gomantong Caves, for example, where nests can be collected only from February to April or July to September), it is not in others, and the swiftlets are declining in areas where the harvest reaches unsustainable levels. 

Some species of birds are also considered nuisances when they nest in the proximity of human habitations. Feral pigeons are often unwelcome and sometimes also considered as a health risk. 

The Beijing National Stadium, principal venue of the 2008 Summer Olympics, has been nicknamed "The Bird Nest" because of its architectural design, which its designers likened to a bird's woven nest. 

In the Victorian era, naturalists often collected bird's eggs and their nests. The study of bird nests is called caliology. 

Artificial bird nests 
Bird nests are also built by humans to help in the conservation of certain birds (such as swallows). Swallow nests are generally built with plaster, wood, terracotta or stucco. 

Artificial nests, such as nest boxes, are an important conservation tool for many species, however nest box programmes rarely compare their effectiveness with individuals not using nest boxes. Red-footed falcons using nest boxes in heavily managed landscapes produced fewer fledglings than those nesting in natural nests, but also than pairs nesting in nest boxes in more natural habitats.
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