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An examination of risk-sensitivity in rufous hummingbirds Paton, Steven Robert


A number of animal species in several major taxa have been shown to be sensitive to both the mean and the variance of prey profitability under laboratory conditions. This type of behaviour is referred to as risk-sensitive foraging because it is thought to minimize the risk of starvation. This thesis consists of three parts. First, I experimentally examine which of two risk-sensitive foraging models best predicts the risk-sensitive foraging behaviour of rufous hummingbirds in the laboratory. Second, I use computer simulations to explore the ability of risk-sensitive foraging to decrease the risk of starvation over risk-insensitive foraging, and to examine the sensitivity of the predictions of one risk-sensitive foraging model to changes in some of its assumptions. Third, I consider several possible explanations for my observations of risk-sensitive foraging behaviour in rufous hummingbirds, even though my computer simulations indicate that such behaviour probably does not significantly reduce the risk of starvation. There are two main classes of models describing how animals should forage when the quality or frequency of food items is variable. I test rufous hummingbird preferences between two alternatives to determine whether rufous hummingbird risk-sensitive foraging preferences are independent of the absolute value of the mean food value. The results contradict the variance discounting models' prediction of constant risk-aversion, and support the z-score model's prediction of decreasing risk-aversion. The z-score model accurately predicts qualitative behaviour, but does not explain several aspects of individual behaviour. I use computer simulations to examine the advantage of risk-sensitive foraging, in terms of decreasing the risk of starvation, by comparing the predicted preferences and estimated probabilities of starvation of a hypothetical risk-sensitive (z-score model) forager with those of a hypothetical risk-insensitive (mean-maximizing) forager. I also examine the sensitivity of the predictions of the z-score model to changes in some of its assumptions. The z-score model seldom significantly reduce the risk of starvation despite the often considerable differences in foraging preferences. Foraging preferences predicted by the z-score model are very sensitive to plausible changes to some of its assumptions. Expected probabilities of starvation are relatively insensitive to major changes in preferences. I consider the apparent paradox that rufous hummingbirds are risk-sensitive even though this foraging strategy probably does not reduce the risk of starvation. My experimental observations of risk-sensitive foraging probably do not represent an experimental artifact. I am unable to analyze the validity of my computer simulations because the assumptions used in the simulations prevent direct comparison to natural situations. The most productive avenue of research in the future may be a comparative study of a range of species which examines possible historical and ecological influences on the observed patterns of risk-sensitive foraging behaviour.

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