EFFECTS OF FORAGING BEHAVIOR AND SPATIAL SCALE ON DIET SELECTIVITY - A TEST WITH FOX SQUIRRELS

We combine the two classical questions of foraging theory, patch use and diet choice, to show how an animal's diet may be the predictable product of simple foraging strategies in response to the distribution and abundance of its foods. We consider four reasons why a forager may exhibit a partially selective diet. First, different foods may co-occur within the same patch and the forager may have different encounter probabilities on different food types. Second, different foods may co-occur within the same patch and the forager may reject an encountered food item based upon its type. Third, different foods may be distributed within distinct patches and the forager's use of a patch may be influenced by joint considerations of food type and abundance. Fourth, different foods may occur in distinct habitats and the forager may allot time among habitats for reasons other than food type. To test these predictions, we measured the giving-up densities of free-living fox squirrels in experimental food parches containing peanut and/or sunflower seeds. When foods co-occur, the squirrels are partially selective on sunflower at high quitting harvest rates, and, at low quitting harvest rates, they are partially selective on peanuts. This switch in selectivity suggests that sunflower are preferred to peanut, and that squirrels have a higher encounter probability on peanut than sunflower. When foods occur in different patches, squirrels are partially selective on whichever food has the relatively higher initial abundance. In response to predation risk, squirrels forage food patches more thoroughly near cover than away from cover. As a result, if peanut and sunflower occur in different habitats, fox squirrel are partially selective on whichever food occurs in the safer habitat.