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 foodencounter rate, and predation risk on cache and fat-storageregulation using dynamic programming. Previous predictionsthat small birds facing increased pilferage rates should cacheless 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 pilferagerates. This pattern is determined, in part, by pilferage-inducedchanges in time budgets: at low pilferage rates, a marginalincrease in pilferage rates can be offset by an increase incache investment. However, increased caching increases time allocated to both caching and foraging. The increased foragingis caused by the energetic costs of caching and by the lossof energy from the cache. Increased time spent caching andforaging in turn decreases time spent resting under low predationrisk. Above some threshold pilferage rate, the marginal valueof resting exceeds the marginal value of caching, and cacheinvestment declines with further increasing pilferage rates.These patterns hold for three levels of variation in food encounterrate: 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-storagelevels with increased food abundance are not supported when there is between-day variation in mean food encounter ratesand food abundance increases only on "good" days. Finally,predation risk affects the predictions described above in twoways. First, these trends assume that the birds can rest ina predator-free refuge. If the refuge is not available, birdsare predicted to cache less at higher pilferage rates irrespectiveof the absolute level of pilferage. With the refuge in place,levels of predation risk affect the skew in the pilferage-rate/cachingfunction. As a result, the relative effect of predation riskon caching intensity varies with pilfer rate. At very low pilferrates, lowered predation risk causes more caching, but loweredpredation risk under high pilferage rates can lower caching intensity, contrary to previous predictions. Surprisingly, predationrisk has an appreciable effect on body mass only when the birdis predicted to cease caching (i.e., at the highest pilferrates); otherwise a change of two orders of magnitude in theprobability of encountering predators has little effect on body mass. Our results suggest that the tradeoffs associatedwith the joint regulation of internal energy stores and externallycached stores are more complicated than previous literaturewould indicate. Our results also show that we have underestimatedthe role that time budgets play in patterns of energy regulation.