The Starvation-Predation Trade-Off And Some Behavioural and Ecological Consequences

There are a variety of ways in which an animal may increase the amount of food it obtains at the expense of an increase in predation risk. I give some examples: (a) The most obvious way to increase the amount of food gained is to increase time spent foraging. Since foraging will typically be a more risky activity than, say resting. An increase in time spent foraging will increase the probability of being taken by a predator. (b) Some areas of food give a higher yield but are more risky (Sih, 1980,1982). (c) Decreasing the level of vigilance will increase the rate of gain but increase predation risk (Pulliam, Pyke & Caraco, 1982; Milinski & Heller, 1978). (d) A bird can decrease its probability of starvation by carrying extra fat reserves. Lima (1986) and McNamara & Houston (in press, a) consider two different mechanisms by which increasing fat reserves, and hence body mass, increases predation risk. Increasing body mass results in increased metabolic costs, especially for flight. This means that more food must be obtained to maintain body weight and hence more time must be spent foraging. Thus the bird is exposed to predators for a greater time each day (cf (a)). Increasing body mass may also decrease a bird’s manoeverability and hence increase the bird’s chances of being taken by a predator.