Influence of predation risk on diet selection: a simple example in the grey squirrel

Current models of the optimal diet are special cases of a more general (and complex) model which incorporates the effects of predation risk on diet selection; this follows from an assumption implicit in current models that all prey items are eaten where they are encountered. Relaxing this assumption so that a forager might carry a prey item to protective cover for consumption leads to the conclusion that the value of a prey item is a function of its distance to cover as well as its energy content and handling time. Such considerations can significantly alter the outcome of diet selection relative to that expected from simple diet theory. We found that grey squirrels (Sciurus carolinensis) may reject more energetically profitable, but small food items in favour of locating larger, less energetically profitable items that can be carried to protective cover for consumption without greatly sacrificing foraging efficiency. The squirrel's tendency to reject a more profitable item is a function of its distance from cover and the size of the less profitable items. Such behaviour is inconsistent with predictions of current diet models, but is consistent with our qualitative predictions based on a previously determined predation-risk-foraging-efficiency trade-off in the grey squirrel.