Aggregation size and foraging behavior of White Storks

The foraging behaviour of breeding White Storks Ciconia cinconia was studied at a colony in central Spain to test various predictions related to the advantages of flocking. Storks devoted more time to preening and resting with increasing aggregation size. Time spent vigilant by individual Storks decreased significantly as aggregation size increased, mainly due to a reduction of scan duration. Locomotion pattern changed significantly with aggregation size: birds in flocks walked head-up less frequently than solitary individuals. Absolute food intake rate was higher in larger aggregations. This was mainly attributable to lower vigilance intensity as aggregation size increased. Net feeding rates were also higher in larger aggregations. Aggregation size increased with distance to the colony. This was interpreted as a trade-off between travel and vigilance times in order to maximize foraging time. Aggregation of foraging Storks was explained both as a response to predation pressure and as a strategy to improve foraging efficiency through local enhancement.

Carrascal L.M., Alonso J.C. & Alonso J.A, ARDEA 78 (3): 399-404