Avian flocking in the presence of a predator

Abstract

Although there are several possible advantages of flocking^1–3, many authors suggest that birds forage in groups to reduce the risk of predation (see citations in ref. 4). One version of the ‘many eyes’ hypothesis proposes that flocking allows individuals to spend less time scanning for predators and more time feeding^5,6. However, flocking may also cause individuals to lose time and energy in fighting one another. The way a bird divides its time among these activities, its time budget, may depend on variables governing foraging requirements and the chance of predation⁷. As one such variable is the frequency of attacks by predators⁸, we flew a trained hawk over flocks of granivorous yellow-eyed juncos (Junco phaeonotus) to compare time budgets in the presence and absence of a predator. We found that time budgets changed after the predator was introduced and also that flock size increased in the presence of the predator.