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Current theory of risk-sensitive foraging predicts that foragers should choose feeding sites on the basis of variation in as well as mean reward rate when there is a shortfall in their food supply or a decrease in their energy budget. For a given mean reward delay, they should choose high variance feeding sites if they are running below energy requirement, but low variance sites if they are running above. It has been suggested that the smaller the animal size, the stronger the preference reversion between high and low variable feeding sites. Previous tests of the energy budget rule when there was time variability have used bird species heavier than 80 g. Hence we tested energy budget rule predictions with coal tits Parus ater, a bird of 9 g of body mass, foraging at two feeding sites with high or low variability in food delivering delay. We manipulated energy budgets by controlling air temperature in the laboratory. In one treatment (positive budget), individuals were allowed to eat at the level of their own ad-libitum daily consumption and the air temperature was set to 24°C, while for the other treatment (negative budget), temperature was set to 14°C, and food availability was limited to the maximum daily intake observed in the positive budget treatment. When air temperature was low, daily intake increased but body mass decreased. Birds were also less active in the low temperature treatment, hopping less times every day. Latency to peck decreased as well, pecking for food when it was available sooner than in the high temperature treatment. These results show that coal tits were living in a negative energy budget when air temperature was set to 14°C. Preference for the variable feeding site was greatest in the negative energy budget, as it was predicted by the energy budget rule. Contrary to the energy budget rule, coal tits consistently preferred the variable option also in the positive energy budget. Possible explanations for these results are explored, including alternative foraging models to the energy budget rule.

Affiliations: 1: Departamento de Ecología Evolutiva, Museo Nacional de Ciencias Naturales, CSIC, José Gutierrez Abascal 2, 28006 Madrid, Spain


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