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

We studied the foraging decisions of six individually marked white sto rks at a breeding colony in central Spain. Storks behaved as central p lace foragers, gathering in flocks to feed. Travel distance and patch residence time were positively correlated with the size of the foragin g flock, but not with patch quality as estimated by the instantaneous food intake rate at the patch. Parch residence time was also positivel y correlated with travel distance to the patch, flock size being const ant. In larger flocks individuals benefitted from a decrease in vigila nce time and thus an increase in time spent actively feeding, which en abled birds to bring back to the nest a higher load size. These result s suggest that storks followed simple rules of thumb based on flock si ze, 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 c ould be a simple rule for many other flock foraging species.