Vigilance and diet selection: a simple example in the dark-eyed junco

Abstract

A straightforward prediction of "classical" optimal diet theory is that an animal should consume only the more profitable of two prey types (in terms of energy/time) in an environment where search time is negligible. However, such a simple diet is not necessarily expected if an animal feeds under the risk of predation. If an animal chooses to be vigilant for predators, and if the degree to which prey handling and vigilance are mutually exclusive varies positively with prey profitability, then the optimal diet may be a mixture of both prey types. The proportion of the mixed diet consisting of the less profitable item should decrease with an increase in both the size of the feeding group and the absolute profitability of the more profitable item, reflecting a trade-off between the need for a high rate of energy intake and the need to be vigilant for predators. Such behaviour was observed in free-living dark-eyed juncos (Junco hyemalis) feeding in simple two-prey environments.