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. Some birds, including magpies, have been observed building nests using anti-bird spikes. In some cases, these nests can contain up to 1,500 metal spikes. Magpies use the spikes to form a protective dome, which may help deter predators and safeguard their chicks, ironically using the spikes in a way that still serves their original purpose of keeping birds away. The smallest bird nests are those of some hummingbirds, tiny cups which can be a mere across and high. At the other extreme, some nest mounds built by the dusky scrubfowl measure more than in diameter and stand nearly tall. The study of birds' nests is known as caliology or nidology.
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 that are used only for roosting. Most birds build a new nest each year, though some refurbish their old nests. The large eyries of some eagles are platform nests that have been used and refurbished for several years. The Eurasian coot also reuses nesting sites, particularly in urban areas like the canals of Amsterdam, where nests made from plastic waste have formed stratified layers over decades. These layers, preserved due to the non-degradable nature of plastic, can be dated using expiration dates on food packaging found within them.
In the majority of nest-building species the female does most or all of the nest construction, in others both partners contribute; sometimes the male builds the nest and the hen lines it. 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.
Type
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, 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.
Scrape
The simplest nest construction is the scrape, which is merely a shallow depression in soil or vegetation. This nest type, which typically has a rim deep enough to keep the eggs from rolling away, is sometimes lined with bits of vegetation, small stones, shell fragments or feathers. These materials may help to camouflage the eggs or may provide some level of insulation; they may also help to keep the eggs in place, and prevent them from sinking into muddy or sandy soil if the nest is accidentally flooded. Ostriches, most tinamous, many ducks, most shorebirds, most terns, some falcons, pheasants, quail, partridges, bustards and sandgrouse are among the species that build scrape nests.Eggs and young in scrape nests, and the adults that brood them, are more exposed to predators and the elements than those in more sheltered nests; they are on the ground and typically in the open, with little to hide them. The eggs of most ground-nesting birds are cryptically coloured to help camouflage them when the adult is not covering them; the actual colour generally corresponds to the substrate on which they are laid. Brooding adults also tend to be well camouflaged, and may be difficult to flush from the nest. Most ground-nesting species have well-developed distraction displays, which are used to draw potential predators from the area around the nest. Most species with this type of nest have precocial young, which quickly leave the nest upon hatching.
File:Peregrine falcon nest-scraping, Derby Cathedral.webm|thumb|Female peregrine falcon nest-scraping on artificial ledge on Derby Cathedral. Both sexes contribute to the creation of a bare, shallow depression in soil or gravel.
In cool climates, the depth of a scrape nest can be critical to both the survival of developing eggs and the fitness of the parent bird incubating them. The scrape must be deep enough that eggs are protected from the convective cooling caused by cold winds, but shallow enough that they and the parent bird are not too exposed to the cooling influences of ground temperatures, particularly where the permafrost layer rises to mere centimeters below the nest. Studies have shown that an egg within a scrape nest loses heat 9% more slowly than an egg placed on the ground beside the nest; in such a nest lined with natural vegetation, heat loss is reduced by an additional 25%. The insulating factor of nest lining is apparently so critical to egg survival that some species, including Kentish plovers, will restore experimentally altered levels of insulation to their pre-adjustment levels within 24 hours.
File:Pluvialis dominica eggs and nest.jpg|thumb|left|alt=Four beige eggs, heavily speckled with black, sit in a shallow depression lined with pale greenish-white lichen.|Other nest linings, like the lichen in this American golden-plover scrape, may provide some level of insulation for the eggs, or may help to camouflage them.
In warm climates, such as deserts and salt flats, heat rather than cold can kill the developing embryos. In such places, scrapes are shallower and tend to be lined with non-vegetative material, which allows convective cooling to occur as air moves over the eggs. Some species, such as the lesser nighthawk and the red-tailed tropicbird, help reduce the nest's temperature by placing it in partial or full shade. Others, including some shorebirds, cast shade with their bodies as they stand over their eggs. Some shorebirds also soak their breast feathers with water and then sit on the eggs, providing moisture to enable evaporative cooling. Parent birds keep from overheating themselves by gular fluttering while they are incubating, frequently exchanging incubation duties, and standing in water when they are not incubating.
The technique used to construct a scrape nest varies slightly depending on the species. Beach-nesting terns, for instance, fashion their nests by rocking their bodies on the sand in the place they have chosen to site their nest, while skimmers build their scrapes with their feet, kicking sand backwards while resting on their bellies and turning slowly in circles. The ostrich also scratches out its scrape with its feet, though it stands while doing so. Many tinamous lay their eggs on a shallow mat of dead leaves they have collected and placed under bushes or between the root buttresses of trees, and kagus lay theirs on a pile of dead leaves against a log, tree trunk or vegetation. Marbled godwits stomp a grassy area flat with their feet, then lay their eggs, while other grass-nesting waders bend vegetation over their nests so as to avoid detection from above. Many female ducks, particularly in the northern latitudes, line their shallow scrape nests with down feathers plucked from their own breasts, as well as with small amounts of vegetation. Among scrape-nesting birds, the three-banded courser and Egyptian plover are unique in their habit of partially burying their eggs in the sand of their scrapes.
Mound
Burying eggs as a form of incubation reaches its zenith with the Australasian megapodes. Several megapode species construct enormous mound nests made of soil, branches, sticks, twigs and leaves, and lay their eggs within the rotting mass. The heat generated by these mounds, which are in effect giant compost heaps, warms and incubates the eggs. The nest heat results from the respiration of thermophilic fungi and other microorganisms. The size of some of these mounds can be truly staggering; several of the largest—which contain more than of material, and probably weigh more than 50 tons —were initially thought to be Aboriginal middens.In most mound-building species, males do most or all of the nest construction and maintenance. Using his strong legs and feet, the male scrapes together material from the area around his chosen nest site, gradually building a conical or bell-shaped pile. This process can take five to seven hours a day for more than a month. While mounds are typically reused for multiple breeding seasons, new material must be added each year to generate the appropriate amount of heat. A female will begin to lay eggs in the nest only when the mound's temperature has reached an optimal level.
Both the temperature and the moisture content of the mound are critical to the survival and development of the eggs, so both are carefully regulated for the entire length of the breeding season, principally by the male. Ornithologists believe that megapodes may use sensitive areas in their mouths to assess mound temperatures; each day during the breeding season, the male digs a pit into his mound and sticks his head in. If the mound's core temperature is a bit low, he adds fresh moist material to the mound, and stirs it in; if it is too high, he opens the top of the mound to allow some of the excess heat to escape. This regular monitoring also keeps the mound's material from becoming compacted, which would inhibit oxygen diffusion to the eggs and make it more difficult for the chicks to emerge after hatching. The malleefowl, which lives in more open forest than do other megapodes, uses the sun to help warm its nest as well—opening the mound at midday during the cool spring and autumn months to expose the plentiful sand incorporated into the nest to the sun's warming rays, then using that warm sand to insulate the eggs during the cold nights. During hot summer months, the malleefowl opens its nest mound only in the cool early morning hours, allowing excess heat to escape before recovering the mound completely. One recent study showed that the sex ratio of Australian brushturkey hatchlings correlated strongly with mound temperatures; females hatched from eggs incubated at higher mean temperatures.
Flamingos make a different type of mound nest. Using their beaks to pull material towards them, they fashion a cone-shaped pile of mud between tall, with a small depression in the top to house their single egg. The height of the nest varies with the substrate upon which it is built; those on clay sites are taller on average than those on dry or sandy sites. The height of the nest and the circular, often water-filled trench which surrounds it help to protect the egg from fluctuating water levels and excessive heat at ground level. In East Africa, for example, temperatures at the top of the nest mound average some cooler than those of the surrounding ground.
The base of the horned coot's enormous nest is a mound built of stones, gathered one at a time by the pair, using their beaks. These stones, which may weigh as much as 450 g each, are dropped into the shallow water of a lake, making a cone-shaped pile which can measure as much as at the bottom and at the top, and in height. The total combined weight of the mound's stones may approach 1.5 tons. Once the mound has been completed, a sizable platform of aquatic vegetation is constructed on top. The entire structure is typically reused for many years.