Foraging Routines of Small Birds in Winter: A Theoretical Investigation

We investigate the way in which the opposing forces of starvation and predation combine to influence daily foraging routines in populations of small, wintering birds. To avoid starvation in an unpredictable foraging environment, feeding should take place early in the day to accrue maximal energy reserves. However, a bird attempting to minimize its risk of predation should delay building up its energy reserves until late in the day. We develop dynamic programming models indicating that the compromise between these opposing forces depends upon the nature of environmental stochasticity in food supply. Foraging may peak early in the day and then decrease steadily if (i) interruptions in foraging are not possible, and (ii) the energetic gain from foraging is relatively poor. With a higher energetic gain, or the possibility of interruptions in foraging, a general foraging routine emerges, with peaks of activity around dawn and dusk. The basic bimodal routine reflects an overt compromise between the avoidance of both predation and starvation, with some feeding taking place during both the early and late portions of the day. Such bimodal routines are observed in many small, wintering birds, but explanations for them have typically focused not on starvation-predation trade-offs, but on daily temporal patterns in food availability or temperature. Our models show not only that bimodal routines emerge in static environments, but also that they are robust to time-dependent environmental variation, and may persist, for instance, even if food is maximally available during the middle of the day. There are currently not enough systematic empirical studies of foraging routines to provide a thorough test of our predictions, but available evidence suggests general agreement between theory and observation.