A dynamic model of short-term energy management in small food-caching and non-caching birds

The survival of small birds in winter is critically dependent on the birds' ability to accumulate and maintain safe levels of energy reserves. In some species, food caching facilitates energy regulation by providing an energy source complementary to body fat. We present a dynamic optimization model of short-term, diurnal energy management for both food-caching and non-caching birds in which only short-day, winter conditions are considered. We assumed that birds can either rest, forage and eat, forage and cache, or retrieve existing caches (the two latter options are available only to caching birds). The model predicted that when (here is variability in foraging success there modeled strictly as within-day variability), both caching and non-caching birds should increase their fat reserves almost linearly in the morning slowing down toward late afternoon, a result consistent with field data but different than the result of a previous dynamic program. Non-cachers were predicted to carry higher fat levels than cachers especially when the variability in foraging success is high. Probability of death for non-caching birds was predicted to be higher than that for cachers, especially at higher levels of variability in foraging success. Among caching birds, an increase in number of caches and fat reserves was also predicted if: (1) mean foraging success was decreased, (2) variability in foraging success was increased, and (3) energy expenditure at night was increased over our baseline conditions. Under the conditions simulated in our model, birds were predicted to cache only if cache half-life (i.e., time interval over which 50% of die caches are forgotten or lost to pilferage) exceeded 2.5 days, indicating that low pilferage rate and long memory fa-vor more caching. Finally we showed that such daily patterns of energy management do not necessarily require relaxing assumptions about mass-dependent predation risk.