Preroosting behaviour of hooded crows (Corvus corone cornix)

Gatherings of birds at preroosts before departure to nocturnal roosts have rarely been studied and their function is poorly understood. We recorded preroosting behaviour of hooded crows, Corvus corone cornix, in a mixed farmland-forest landscape in southeastern Norway during fall and winter, by selecting gatherings attended by at least one radio-tagged crow for observation. The preroosting crows perched in a highly aggregated and exposed group and did not forage. As the amount of snowfall during the preceding 24 h increased, an indication of sudden difficulty in finding food, the maximum number of crows that gathered increased, the duration of the gathering increased, and the straight-line distance covered by the radio-tagged crows from preroost to roost increased. The crows flew conspicuously in formation more often and for a longer time when on their way from preroost to roost on days with stronger winds. The results suggest that, in hooded crows, a preroost gathering is not simply a consequence of many individuals approaching the same roosting area but has some function, presumably that of maximizing the probability of finding rewarding food sites the next day by finding is a communal roost where information on the location of these sites may be obtained.