A Long-Term Approach to Foraging Optimization

A long-term approach to optimal foraging strategy in response to a regularly varying environment is presented. A dynamically optimal animal uses this regularity to produce the best strategy vis-a-vis a single long-range objective. (No long-range planning ability need be postulated, however. There need only be selection for behaving via environmental cues as if planning occurred.) This dynamic optimization approach is applied to a model of the African weaver bird Quelea quelea (documented by Ward 1965a, 1965b, 1971). This model includes the effect upon the animal's condition of past and present behavior and variations (with time) in environmental and foraging characteristics, but contains a number of simplifications: (1) considering food availability only in terms of calories, ignoring other differences between foods; (2) ignoring the distinction between components (e.g., skeleton, fat) of body weight; (3) assuming that Quelea's sole control over its food energy input is its daily feeding time, which implicitly assumes that foraging characteristics such as capture efficiency are either preordained or already optimized in the short run; (4) assuming linear relationships between many variables, for mathematical simplicity. The model, with parameters and assumptions chosen to match qualitatively the food availability and time-energy demands of actual animals, optimizing a long-range goal of minimizing total annual feeding time, produces a feeding strategy (decision variable as a function of time) and corresponding weight history (animal's condition as a function of time) bearing good qualitative resemblance to those of the actual animal. Some features of the model's feeding strategy and weight history (e.g., weight decrease in the lean season) are foreordained by assumptions on the model's parameters. However, the following features (whose purposes may not be obvious, viewed only in the short run) give insight into trade-offs between tactics vis-a-vis the long-range goal: (1) nonequivalence of the feeding strategies for minimizing daily feeding time each day and for minimizing total annual feeding time; (2) more specifically, inclusion in the optimum strategy of only three types of feeding behavior (maximum, nonfeeding, and minimum maintenance) with abrupt changes from one to the next, so that the (actual animal's) sharp beginning of the first weight peak is optimal as well as implementable from environmental cues; (3) concentration of as much feeding as possible in the lush season. The trade-off between reproductive effort (effort in feeding the young, which limits surviving clutch size) and adults' weight gain is considered in choosing between two versions of optimum weight histories. Generalizations are outlined to include wider classes of strategy choices and goals, including territoriality, migration, multiple feeding modes, molting, risk minimization, weight (energy) maximization, and weight-risk combinations.