Diet selection in birds: trade-off between energetic content and digestibility of seeds

Partial preferences may occur due to differences in profitability and encounter probability between food types within a patch. In the expanding specialist diet strategy, a forager goes from being partially selective on the preferred, most profitable food, to being opportunistic, after the preferred food has been depleted to a certain critical level. We studied the diet selection strategies of birds foraging on 2 seed types--crushed peanut Arachis hypogaea and husked millet Pennisetum gambiense, in a woodland savannah near Jos, Nigeria. Peanut contains more fat and energy, whereas millet is richer in carbohydrates. We tested 2 hypotheses: 1) energy content will determine seed preference for seeds with relatively similar handling times and 2) diet selection will vary seasonally in response to nutritional demands. We carried out 2 experiments, one with both seed types occurring in a single patch and another with both occurring in 2 separate patches during different seasons. The diet selection strategy of the birds was similar to the expanding specialist. Initially, peanuts seemed to be the preferred food, probably due to their high profitability, but overall millet was preferred. However, the expansion point was not determined by the amount of peanut seeds left, as predicted, but rather by the amount eaten. We propose that a trade-off is created by the fact that peanut has higher energy density but at the same time contains secondary compounds. Selectivity for millet decreased slightly in the wet season when more peanuts were taken possibly due to increasing nutritional demands during breeding. Copyright 2011, Oxford University Press.