Load size and energy delivery in birds feeding nestlings: Constraints on and alternative strategies to energy-maximization

Abstract

The Orians-Pearson model of central place foraging for multiple-prey loaders assumes, as does most of current foraging theory, (i) that the quantity to be maximized is rate of energy delivery, and (ii) that the capacity of the foraging animal is the only relevant constraint on this rate. When applied to the case of birds feeding nestlings the model predicts, therefore, that the parents should select whichever load size maximizes the rate of energy delivery to the young. We assume here (i) that the parents could alternatively maximize the rate of energy delivery to the nest (m-strategy), maximize the time available for activities other than foraging (e.g. brooding) (b-strategy), or minimize the frequency of visits to the nest (v-strategy). We further assume (ii) that the nestlings impose constraints on the foraging behaviour of their parents in that there is a maximum load size that a brood can receive, as well as a maximum and a minimum rate of energy delivery that is acceptable. These three quantities are increasing functions of the age of the nestlings. Load size as a function of the age of the nestlings is predicted to increase initially and then to approach either the load size corresponding to maximum parental efficiency (m- and b-strategies) or a larger load size (v-strategy). For the b- and v-strategies rate of energy delivery is predicted to correspond to the minimum requirement of the nestlings throughout the nestling period. For the m-strategy energy delivery is predicted to be the maximum energy requirements of the nestlings initially, and then to level off to the energy delivery rate that corresponds to the maximum parental efficiency. As these strategies are not always compatible, foraging behaviour in a particular case may be an adaptive compromise between them.