Central Place Foraging in Starlings (Sturnus vulgaris). II. Food Allocation to Chicks

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Abstract
The allocation of food between self and offspring in birds was studied using central place foraging starlings (Sturnus vulgaris). Two models of the proximate control of short-term parental decisions were developed. The regulation model predicts food allocation assuming that the parent eats the amount of food necessary to compensate its expenditure and gives the excess to the chicks. The LRS model uses assumptions derived from maximization of lifetime reproductive success. The main assumptions are that: (i) current brood productivity is a positive, negatively accelerated function of rate of food provisioning to the nest, and (ii) residual reproductive value of the parent is a positive, negatively accelerated function or rate of parentaly intake. Instantaneous control of food allocation is assumed to maximize the sum of current brood productivity plus residual reproductive value of the parent. The role of biparental food provisioning is discussed. Food allocation was measured in two field experiments using artificial feeding stations. Harvest rate was manipulated by changing the distance between feeder and nest and by modifying the schedule of food delivery at the feeder. The feeder schedules were either a progressive inter-prey interval (experiment 1) or a constant (5 s) inter-prey interval (experiment 2). These modifications resulted in various proportions of flying time and thus modified both harvest rate and parental rate of expenditure. Brood size was manipulated by addition or removal of chicks from the next. Food allocation was sensitive to both harvest rate and brood size, as predicted by the LRS but not by the regulation model. The fit of the quantitative predictions was examined using independent estimates of the parameters. The same parameter values were used for both models and both experiments. These comparisons are shown in Fig. 4 (experiment 1) and Fig. 6 (experiment 2). The LRS model produced a better fit than the regulation model in both experiments, but the fit was only tight for the second experiment. We postulate that the evolutionary thinking embodied in the LRS model helps in understanding and predicting proximate control of allocation decisions. The LRS model has acknowledged shortcomings: it uses rates of intake and provisioning without reference to consequent changes in state of brood and parent, it does not incorporate the finite time horizon imposed by the time of independence of the young and stochasticity is not considered. These shortcomings can be overcome using stochastic dynamic modelling techniques.