Communal Roosting

When flocks of a million or more starlings and blackbirds make pests of themselves by spending the night close to human habitation, the phenomenon of communal roosting comes to public attention. These gigantic swarms, however, hardly approach the record for communally roosting birds. The Red-billed Quelea (a weaver, related to the House Sparrow) is so abundant that it may be considered the avian equivalent of a locust plague when invading African grain fields en masse. Quelea roosts may number tens of millions of individuals; poisons, explosives, and even flame-throwers have been used in attempts to control local outbreaks. The record-holding communal rooster, however, was North American; the now-extinct Passenger Pigeon roosted (and nested) in gigantic colonies containing billions of individuals and covering square miles.

All birds roost -- that is, have a period of inactivity analogous to sleep in human beings. Some birds do it alone; others with mobs of compatriots. Some change their roosting habits with the season: male Red-winged Blackbirds usually roost alone on their territories when breeding, but crowd together at night during the rest of the year. Birds that roost communally do so in a wide variety of situations. Small groups of nuthatches or creepers spend the night together in tree cavities. Some vultures roost on cliffs, and others on the tops of cacti; many seabirds roost on islands, and swallows may roost on telephone lines. Starlings choose an enormous diversity of roost sites -- many kinds of woodlands, cattails and other reeds, and numerous kinds of buildings, to name a few.

The question of why some birds roost communally and others roost solitarily is related to the question of why there are both communal and solitary nesters. One possibility is that older, more experienced birds are better able to find food; hence younger birds roost with them in order to follow their elders to better foraging grounds. The older birds accept this social parasitism because they tend to be dominant, and are able to appropriate more central and therefore safer positions in the roosting crowd. As long as the costs of increased competition are outweighed by the benefits of increased safety from predators for the older birds, and the benefits of locating rich food supplies for the young outweigh reduced nighttime safety for them, roosting should be communal. In fact, some studies have reported that older Red-winged Blackbirds and Brown-headed Cowbirds are concentrated in the centers of their roosts.

In a Mexican mixed-species roost of egrets, herons, and other species, Snowy Egrets and Great Egrets displaced other species from the higher (and presumably safer) positions in the trees; other species have been found to get more food if they forage near Snowy Egrets, and other birds tend to be attracted more to dummies of Snowy Egrets placed in foraging sites than to dummies of other species. No studies have yet been done to determine whether the egrets are actually followed to foraging sites from the mixed-species roosts, however. These observations, and those of the blackbirds and cowbirds, are consistent with the notion that older and younger birds (or species with divergent capacities to locate food) join communal roosts for different reasons.

Further support for this notion comes from observations of swallows in Denmark. Older birds were more successful in finding food than younger ones, and also displaced the youngsters from the safest roosting positions. Evidence was also found that the young swallows were somehow able to evaluate the feeding success of adults and preferentially follow the well-fed ones when they left the roost the next day.

One further advantage can accrue to birds that roost together at night -- they may be able to huddle together to keep warm. For nuthatches that jam together in cavities this might be the main advantage, as they significantly reduce their heat loss. Even in large roosts in which huddling does not occur, more central positions may often be thermally advantageous as a result of denser vegetation (relative to the periphery of the roost) and a greater mass of bird bodies per unit area. Both of these factors may act to reduce the loss of heat from individual birds, primarily by reducing the cooling effects of wind. That reduction, however, would rarely be enough to compensate for the energy lost flying the extra distance to the roosting site. In addition, there is some evidence that birds in the lower positions in colonial roosts lose heat because the rain of droppings from higher birds reduces the insulating properties of their plumage. It thus seems unlikely that thermoregulation is a prime reason for communal roosting in most species.

SEE: Coloniality; Mixed-Species Flocking; Temperature Regulation and Behavior; Commensal Feeding; Flock Defense.

Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.