Social information, antipredatory vigilance and flight in bird flocks

I used a simulation model to study how individual responses to predation risk and vigilance strategies may lead to departure in a foraging bird flock. The model incorporates risk dilution and, to some extent, collective detection of predators. A major factor influencing birds' behaviour is individual perception of predation risk. In the model, this perception is partly based on the observation of the other birds in the flock. It increases when these birds reveal their anxiety by becoming vigilant or taking flight. In general, the model describes birds' departure as a contagious phenomenon, which quickly spreads over the whole group. However, isolated flights involving only a few individuals may also occur. The model predicts that flock size and reliance on social information will strongly affect the moment that birds decide to stop feeding and take flight, even in the absence of any predator attack. In general, birds in small flocks leave the food patch sooner, and having accumulated lower energy reserves, than birds in larger groups. When disturbed, a flock may take flight instantaneously, simply because the high level of vigilance in the group increases anxiety. As a consequence, the model predicts that high levels of disturbance may have important effects on the mean level of energy reserves in the flock. (c) 2006 The Association for the Study of Animal Behaviour Published by Elsevier Ltd. All rights reserved.