A THEORETICAL INVESTIGATION OF THE FAT RESERVES AND MORTALITY LEVELS OF SMALL BIRDS IN WINTER

We assume that the fat levels of a small bird in winter reflect a trad eoff between starvation and predation. This tradeoff is formalized by finding the level of fat that minimizes total mortality (starvation pl us predation) in a given environment. A bird is characterised in terms of its level of energy reserves. The bird starves if these reserves f all to zero. In general, the probability that the bird is killed by a predator increases with increasing reserves. We consider two models. I n both at each time unit during a day a bird can choose, as a function of its reserves, to forage or to rest. In the forage vs rest model th ere is only one foraging option. In the foraging intensity model the b ird can choose from a range of options, where options with a high ener getic gain also incur a high predation risk. We find the optimal level of reserves for various environments, together with the resulting lev els of starvation and predation. Unless food availability is very high , an improvement in overall availability results in decrease in the me an reserves at dusk (MRD); in the foraging intensity model there is al so a trend towards choosing safer foraging options with lower mean gai ns. An increase in variability, either as a result of a decreased prob ability of finding food or an increase in interruptions of foraging, i ncreases MRD. The increase in reserves is not sufficient to prevent an increase in starvation. As a result, reserves and starvation can be p ositively correlated across environmental conditions. The level of sta rvation tends to be lower than the level of predation, but the ratio o f starvation to predation tends to increase as conditions become worse . In the middle of winter, the days arc short and the nights are long and cold. The optimal response to a decrease in daylength involves an increase in both dawn and dusk levels of reserves. This pattern is als o found when overnight expenditure increases but daylength is constant . In this context we show that when temperatures are very low, a small saving in energy can result in a substantial increase in survival pro bability. The relevance of this result for evaluating the importance o f hypothermia is discussed.