A reevaluation of the logic of pilferage effects, predation risk, and environmental variability on avian energy regulation: the critical role of time budgets

We studied the effect of pilferage rates, variation in food encounter rate, and predation risk on cache and fat-storage regulation using dynamic programming. Previous predictions that small birds facing increased pilferage rates should cache less and store more body fat are not generally supported. Instead, cache investment (caching rate or percent of food cached) is predicted to be unimodal, peaking at intermediate pilferage rates. This pattern is determined, in part, by pilferage-induced changes in time budgets: at low pilferage rates, a marginal increase in pilferage rates can be offset by an increase in cache investment. However, increased caching increases time allocated to both caching and foraging. The increased foraging is caused by the energetic costs of caching and by the loss of energy from the cache. Increased time spent caching and foraging in turn decreases time spent resting under low predation risk. Above some threshold pilferage rate, the marginal value of resting exceeds the marginal value of caching, and cache investment declines with further increasing pilferage rates. These patterns hold for three levels of variation in food encounter rate: time-invariant, between-day, and within-day variation; they also hold across different mean rates of food encounter. We show that previous predictions concerning decreased energy-storage levels with increased food abundance are not supported when there is between-day variation in mean food encounter rates and food abundance increases only on good days. Finally, predation risk affects the predictions described above in two ways. First, these trends assume that the birds can rest in a predator-free refuge. If the refuge is not available, birds are predicted to cache less at higher pilferage rates irrespective of the absolute level of pilferage. With the refuge in place, levels of predation risk affect the skew in the pilferage-rate/caching function. As a result, die relative effect of predation risk on caching intensity varies with pilfer rate. At very low pilfer rates, lowered predation risk causes more caching, but lowered predation risk under high pilferage rates can lower caching intensity, contrary to previous predictions. Surprisingly, predation risk has an appreciable effect on body mass only when the bird is predicted to cease caching (i.e., at the highest pilfer rates); otherwise a change of two orders of magnitude in the probability of encountering predators has little effect on body mass. Our results suggest that the tradeoffs associated with the joint regulation of internal energy stores and externally cached stores are more complicated than previous literature would indicate. Our results also show that we have underestimated the role that time budgets play in patterns of energy regulation.