Place chosen or constructed by a bird or other animal for incubation of eggs, hibernation, and shelter. Nests vary enormously, from saucerlike hollows in the ground, such as the scrapes of hares, to large and elaborate structures, such as the 4-m/13-ft diameter mounds of the megapode birds.
Birds' nests Birds that nest in slight depressions in the ground, such as terns and plovers, lay eggs that so closely resemble the ground that they generally escape detection. A slightly more elaborate nest is the burrow, such as that occupied by the sand martin, kingfisher, or puffin. In many cases these underground nests are made in burrows left by rabbits or voles.
Many birds and some animals make their nests in the hollows of trees; the female of the Bornean rhinoceros hornbill is sealed up by the male for many weeks, until her chicks are almost ready to leave the nest. The mud nests of the swallow family and other birds are built mainly with mud; some of these weigh over 2 kg/4.5 lb. Among the crudest nests that are made with collected material, such as sticks, leaves, blades of grass or hair, are those of the wood pigeon, which are so loosely put together that the eggs are visible through them.
Swallows, martins, sparrows, and flycatchers prefer houses as nesting sites; the thrushes, finches, and linnets use evergreen bushes and hedges. Most species construct nests that are highly distinctive.
Other nest-builders Among the fish, good examples of nest-builders are the three-spined stickleback Gasterosteus aculeatus and the Siamese fighting fish Betta splendens. The stickleback's nest is anchored to the river bed and constructed of algae and other aquatic plants stuck together by a secretion from the fish's kidneys. The nest of fighting fish is a floating bubble nest anchored to a piece of plant material into which the eggs are placed by the male after courtship.
Among the insects, nest-builders include the social insects, the ants, bees, and wasps, and their nests or hives, and the huge termite nests, up to 3 m/10 ft tall, found in the tropics.
Of the mammals, good examples are those of the harvest mouse Micromys minutus and the red squirrel Sciurus vulgaris. The harvest mouse nest is a kind of ball made of interlaced leaves of grass that generally belong to the same plants that support it above the ground. The red squirrel's nest is composed of a layer of twigs with a layer of moss or bark fragments. The whole thing is usually placed at the base of a large branch.
Edible nests These are produced by several species of swiftlet found in southeast Asia. The edible nest swiftlet, Collocalia fuciphaga, makes the translucent white nest used in bird's nest soup, long regarded as a delicacy in China. The nests are made from the saliva of the swiftlets and in the nest of this swiftlet there are few impurities. Other species, such as C. maxima, produce a black nest that contains large numbers of feathers and is therefore not regarded so highly. Both species inhabit dark caves and have a special echolocating system that enables them to move around in the dark. The nests are made high up on the walls and the local people climb precarious bamboo ladders and platforms to harvest them.
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