Group foraging by wintering rooks: combined effects of safety and energy requirements

We report the results of feeding experiments with free-ranging gregarious corvids, rooks (Corvus frugilegus), at varying ambient temperatures and risk of interruption. Feeding behaviour depended on the photoperiod and the interaction between ambient temperature and safety of the feeder. Birds did not trade off safety for increasing food intake; instead their behaviour became more risk-averse during short photoperiods or low ambient temperatures, when energy requirements were presumably the highest. We propose that rooks are not severely limited by food supply and, when environmental conditions deteriorate, can quickly increase body reserves. The added fat load, however, handicaps their ability to escape from a danger, thus increasing risk aversion. Only the rooks which initiated foraging were sensitive to safety of the feeder and similar reactions were not observed among birds which joined later. We suggest that rooks, while foraging gregariously, rely on the ‘safety in numbers’ effect, which enhances feeding success. Some other observations also supported this hypothesis. Thus, food intake by small groups of foragers was higher on ‘safe’ than on ‘risky’ feeders, but this pattern was reversed with increasing group size. This suggests that individuals in a large group can reduce scanning for predators and focus their attention on collecting food. An increased feeding rate allows the time exposed to danger to be shortened. Foraging behaviour of birds showed greater synchronisation at risky places than at safe ones. This might be a reaction to the increased costs of feeding, which is predicted by theoretical models of group foraging by selfish individuals.