Supply and Demand in Tree Swallow Broods: A Model of Parent-Offspring Food-Provisioning Interactions in Birds

Abstract

A supply-and-demand model was developed to describe parent-offspring food-provisioning interactions in nidicolous birds. The supply function represents the feeding response of the parents to the hunger signals of the brood, and the demand function describes the hunger-signaling response of the brood to the feeding rate. The intersection of the two functions is an equilibrium point representing the feeding rate and hunger-signaling level observed in the real world. The model was tested for broods of six tree swallows at 13 days of age. It was shown that the supply function shifts in response to changes in food abundance in the environment and that compensating shifts in the demand function occur in accordance with the nutritional condition of the young. The resulting feeding rates are relatively stable over wide ranges of food abundance and hunger-signal level, indicating that the behavioral system enables parents to adjust feeding rates to the needs of the young, in spite of variations in their environment. I postulate that supply and demand functions have genetically determined components molded by natural selection. Shifts in the supply function indicate that there is a proximate effect of food abundance on feeding rate, but these shifts do not support the hypothesis that the food supply in the environment places an absolute evolutionary constraint on clutch size by limiting the ability of parents to feed their broods. The model presented here applies strictly to a single parent with a single offspring. More complex models will be necessary to describe sibling competition and interactions between parents. Supply and demand behaviors clearly form a communication system between parents and their broods, but this study does not indicate whether these behaviors have components that are derived from genetically based parent-offspring conflict.