Communal roosting in birds.
Selection of roosting site is often influenced by species-typical preferences for particular habitats, by proximity to food and water and by isolation from human activity. Birds usually show strong loyalty to a given site. Some species approach roosts in stages that include assembly at feeding grounds, gathering along established flight-lines, assembly in the immediate vicinity of the roost and finally entry into the roost itself. Roost departures typically have fewer assembly rituals during the return to feeding grounds; more birds leave simultaneously and at lower light intensities. The most important factor governing roosting flights is probably illumination level, but other factors (eg temperature, wind, hunger, social stimuli, distance of destination, time of day and time of year) also play roles. On a daily as well as seasonal basis, roost populations fluctuate greatly in total numbers and in age and sex composition. Mixed species roosts are not uncommon, and have implications for survival value. Potential survival values of communal roosting include thermoregulation, protection from predation, increased feeding efficiency, population regulation and preparation for migration. -Author
Eiserer, L. A., "Communal roosting in birds." (1984). Faculty Publications. 1582.