Competing for position in the communal roosts of long-tailed tits

Communal roosting is widespread among social animals and has several hypothesized functions, including reduction of predation risk and thermoregulatory costs, and information sharing. The benefit derived from roosting in close proximity to conspecifics is likely to depend on an individual's position within the roost, but there have been few quantitative studies of the process of communal roost formation and the factors influencing relative positions. We studied the communal roosting behaviour of temporarily captive flocks of long-tailed tits, Aegithalos caudatus, a cooperative breeder that forms flocks during the nonbreeding season. Our objectives were to determine whether (1) individuals compete for particular positions within roosts, (2) individuals achieve consistent positions within roosts, and (3) an individual's roost position is a function of its dominance status. During roost formation birds were more likely to move to inner positions regardless of whether they started in an outer or inner position. However, as the number of birds in the roost increased during roost formation, birds in outer positions were less likely to relocate to an inner position. The same individuals occupied outer positions in a roost on successive nights, but there was no consistency in the occupation of specific inner positions. Birds occupying outer roost positions were significantly less dominant than those occupying inner positions. Our results show that long-tailed tits compete for inner positions within communal roosts, and that an individual's dominance status within a flock influences the outcome of this competition.