The evolution of communal roosting in birds: origin and secondary losses

Three main benefits are thought to underlie communal roosting in birds: a r eduction in thermoregulation demands, a decrease in predation risk, and an increase in foraging efficiency. I investigated interspecific variation in communal roosting tendencies across categories of several ecological factor s to examine the relevance of each functional hypothesis in the evolutionar y transition to communal roosting and the secondary reversal to solitary ro osting habits. The study phylogenetic tree included 30 families and 437 spe cies. Evolutionary transitions to communal roosting occurred more often on branches with flocking species and with larger species but were not associa ted with diet, territoriality, geographical area, or time of day. The assoc iation with flocking activities suggests that increased foraging efficiency , a factor thought to operate through the formation of flocks, may have bee n a key factor in the origin of avian communal roosting. However, several t ransitions to communal roosting occurred on branches with nonflocking speci es, indicating that foraging efficiency may not be the only factor involved in the evolution of communal roosting. Secondary losses of communal roosti ng habits occurred on several branches, with a concomitant loss of flocking behavior and a tendency to exhibit territorial behavior and nocturnal fora ging. Secondary losses suggest that communal roosting is costly to perform and maintain and may be lost when an asocial selection regime operates. The large number of exceptions to the above patterns may force a reevaluation of current functional hypotheses about communal roosting in birds.