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 (0.8 in) across and 2–3 cm (0.8–1.2 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. The study of birds' nests is known as caliology.
Deep cup nest of the great reed-warbler
Some nest linings, such as the shell fragments in this Charadrius plover scrape, may help to prevent the eggs from sinking into muddy or sandy soil.
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.
The huge mound nest of the malleefowl acts like a compost heap, warming and incubating the eggs as it rots around them.
Structures built by animals
Structures built by non-human animals, often called animal architecture, are common in many species. Examples of animal structures include termite mounds, ant hills, wasp and beehives, burrow complexes, beaver dams, elaborate nests of birds, and webs of spiders.
A so-called "cathedral" mound produced by a termite colony
A young paper wasp queen (Polistes dominula) starting a new colony
Nest, eggs and young of the red-wattled lapwing which depends upon crypsis to avoid detection of its nest
The red-faced spinetail places bits of grass and other material loosely streaming around its nest to break the shape and to masquerade as debris.