Flock density, social foraging, and scanning:: an experiment with starlings

Social foraging differs from individual foraging because it alters both resource availability and the forager's behavior. We examined responses of starlings to the presence of conspecifics by manipulating foraging-group density experimentally, while ensuring that each subject's foraging opportunities were unchanged. To do this, we used individuals foraging simultaneously in four bottomless enclosures placed at various separations in natural foraging grounds. We measured foraging and scanning intensity and qualitative aspects of scanning of focal individuals. Additionally, we examined the temporal distribution of scanning between individuals. The focal individual analysis showed that (1) food-searching activity increased, while time spent scanning, time off the ground and scanning bout length decreased with flock density; (2) food finding per unit of searching effort increased with density; (3) head orientation during scanning was sensitive to companions' proximity: heads pointed away from the companions at close distance, toward them at intermediate distance, and was random farther away. The analysis of the (temporal overlapping in scanning) temporal distribution of scanning for the group showed that scanning was significantly synchronized when companions were adjacent to each other but was not significantly different from random at further separations. We conclude that behavioral responses of individuals to the presence of others generate important changes in foraging performance even in the absence of physical interference and, more generally, that assessing the mechanisms that control the behavior of group members at different flock densities offers a way to understand the functional and ecological significance of foraging aggregations.