Profitability, Rate Maximization, and Reward Delay: A Test of the Simultaneous-Encounter Model of Prey Choice with Parus atricapillus

Abstract

Recently, several authors attempted to generalize the optimal-diet model by relaxing the assumption of the sequential encounter of prey. Engen and Stenseth (1984) and Stephens et al. (1986) showed that, for some prey characteristics, diet models incorporating the simultaneous encounter of different prey types make strikingly different predictions from those of the traditional diet model. In particular, when the less profitable prey is larger than the more profitable prey, a long-term rate-maximizing predator is predicted to specialize on the larger but less profitable prey if the encounter rate is sufficiently low. We tested this prediction with captive black-capped chickadees. The results were inconsistent with the prediction of the model. The chickadees always preferred the higher-profitability prey, regardless of encounter rate. A second experiment was designed to determine the proximate factor affecting the chickadees' choice in experiment I. The chickadees could have been choosing prey so as to minimize pursuit time or to maximize profitability; the results indicated the latter. We suggest that, contrary to what has previously been assumed, foragers often choose prey to maximize profitability rather than long-term intake rate, the traditional currency of optimality models of foraging. This might have been obscured by the empirical emphasis on testing the sequential-encounter diet model. The sequential-encounter case does not present foragers with a clearly dichotomous choice between maximization of short-term and long-term intake rates, whereas the simultaneous-encounter case does. Factors previously thought to affect foraging choice need closer examination. We think that the most parsimonious explanation of our results is to reject a diet model based solely on a currency of long-term rate maximization. We consider several alternatives, concluding that a model of prey choice based on maximizing profitability that incorporates discounted future rewards can explain a variety of empirical results, including our own.