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 pr eroosting behaviour of hooded crows, Corvus corone cornix, in a mixed farml and-forest landscape in southeastern Norway during fall and winter, by sele cting gatherings attended by at least one radio-tagged crow for observation . The preroosting crows perched in a highly aggregated and exposed group an d did not forage. As the amount of snowfall during the preceding 24 h incre ased, an indication of sudden difficulty in finding food, the maximum numbe r of crows that gathered increased, the duration of the gathering increased , and the straight-line distance covered by the radio-tagged crows from pre roost to roost increased. The crows flew conspicuously in formation more of ten 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 s ame roosting area but has some function, presumably that of maximizing the probability of finding rewarding food sites the next day by finding is a co mmunal roost where information on the location of these sites may be obtain ed.