Analyzing foraging and habitat use through selection functions

Methods commonly used for study of natural selection in changing populations are useful in the evaluation of food and habitat selection. In particular, one can derive “selection functions” that allow estimation of the relative probability that a given food item (or habitat class) will be selected next, given equal access to the entire distribution ofavailable food items (or habitat points). Procedures are available for the cases when food items are assigned to qualitative classes and when the items are characterized by measurement of quantitative variables on the item. The primary advantage of these analysis methods is that clear probabilistic statements can be made concerning the likelihood that each of several food or habitat types will be used. Illustrations are given by analysis of prey size selection by Wilson’s Warblers ( Wilsonia pusilla) and by Tree Swallows (Tachycineta bicolor).