THE NEST AS A DORMITORY

Summary As a rule, only birds which breed in roofed nests or those built in enclosed spaces sleep in their nests when not incubating or brooding young. The chief exceptions are rails, coots, and gallinulse, which build platforms to raise them above the water or wet ground while they sleep, and Curved‐billed Thrashers, which roost on open nests in thorny cacti. The first step in the evolution of the dormitory habit appears to be sleeping in the covered breeding nest itself before the eggs are laid or after the fledglings depart. In the simplest case, only one parent occupies the breeding nest. But in a number of species, the second parent uses the nest as a dormitory even while it contains eggs and nestlings, and sometimes unmated helpers roost with the parents. A further advance is made when the fledglings are led to sleep in the nest they have just left. The next advance is the construction of special dormitories, not intended for eggs. These usually resemble the breeding nest, but differences in form or site are sometimes found, especially in tropical wrens. Only birds which make or acquire dormitories, in addition to their breeding nests, are likely to enjoy such lodges through the year. In them, self‐supporting individuals may sleep singly, or in pairs, or with their dependent offspring. In the final stage of the evolution of the dormitory habit, the young lodge with their parents long after they are self‐supporting. Often each family remains separate throughout the year; but sometimes, probably in consequence of the loss of dormitories, several families club together in the non‐breeding season. Numerous examples of each of the foregoing arrangements are given. In a harsh environment, as at high latitudes and above timberline on tropical mountains, many birds seek closed spaces for sleeping, and their lives may depend on the availability of such protection. Nevertheless, the dormitory habit is best developed in mild climates where the birds are permanently resident.