Predation Risk and Unpredictable Feeding Conditions: Determinants of Body Mass in Birds

The body mass (or fat reserves) maintained by a wintering bird can be viewed as a trade—off between the risk of starvation and the risk of predation. This follows from the fact that fat reserves after survival in very different ways. From the starvation point of view, a bird should be as fat (or heavy) as possible in order to minimize its probability of starvation during weather—related food unavailability because fatter birds can survive longer without food than leaner birds. From the predation risk point of view, however, a bird should be as lean as possible to minimize its probability of being killed. Leaner birds will incur a smaller cost of existence than heavier birds and thus spend less time feeding, potentially exposed to predators. In addition, lean birds are likely to be more adept at escaping predators once attacked. If a bird is attempting to minimize its probability of death during the winter season, the fat reserves and thus the body mass it maintains will reflect a trade—off between starvation and predation risk. A simple stochastic simulation model developed to explore the nature of this trade—off indicates that body mass should: (1) increase with the frequency and/or harshness of periods of food unavailability (due to snow, ice, wind, etc.); (2) decreases with increasing predation risk; (3) decrease with increasing temperature; and (4) increase with food abundance. The analysis of the model readily explains why wintering birds maintain fat levels lower than those of which they are capable. The tendency for individuals of a given bird species to be heavier in more northerly populations and during midwinter is also readily interpreted in terms of the above trade—off. Implications for foraging behavior and population regulation in wintering birds are discussed.