bird nest

Deep cup nest of the great reed-warbler

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 iself—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 (other) birds away. The smallest bird nests are made by some hummingbirds; these tiny cups 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 or nidology.