FLOCK SIZE AND FORAGING DECISIONS IN CENTRAL PLACE FORAGING WHITE STORKS, CICONIA-CICONIA

We studied the foraging decisions of six individually marked white storks at a breeding colony in central Spain. Storks behaved as central place foragers, gathering in flocks to feed. Travel distance and patch residence time were positively correlated with the size of the foraging flock, but not with patch quality as estimated by the instantaneous food intake rate at the patch. Parch residence time was also positively correlated with travel distance to the patch, flock size being constant. In larger flocks individuals benefitted from a decrease in vigilance time and thus an increase in time spent actively feeding, which enabled birds to bring back to the nest a higher load size. These results suggest that storks followed simple rules of thumb based on flock size, rather than on the more complicated food availability estimations required by central place foraging models. We suggest that flock size could be the main cue used in foraging decisions by white storks and could be a simple rule for many other flock foraging species.