Temporal, spatial and social segregation of red-billed choughs between two types of communal roost: a role for mating and territory acquisition

We assessed the value of communal roosting for mating and territory acquisition in nonbreeding red-billed choughs, Pyrrhocorax pyrrhocorax. Between 1991 and 1996 we surveyed all communal roosts, breeding territories and vacant nest sites in an area of 250 km(2) in Los Monegros (northeast Spain), as well as other communal roosts in the surrounding 500 km(2) from 1987 to 1996. Main roosts were used traditionally and throughout the year and gathered more choughs than subroots, which were used only during the nonbreeding season. Nonbreeding choughs were socially segregated in these roosts according to their age and breeding prospects. Compared with random potential roost sites, main roosts included more first-year birds and were in areas of higher breeding density. Subroosts were mainly joined by choughs of breeding age (at least 2 years old) whose morphology (males) and body condition (females) were similar to those of established breeders, and were in areas with breeding densities as high as around main roosts but with more available nest sites. Nonbreeding choughs that used main roosts later used subroosts, while the converse was rare. More choughs joining subroosts acquired mates than those joining main roosts. Choughs mostly paired with roostmates but also with widowed territorial breeders. Finally, choughs from subroosts acquired territories closer to the roost than those who mated in main roosts, supporting the hypothesis that subroosts reduce the costs of mating and territory acquisition. (C) 1999 The Association for the Study of Animal Behaviour.