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Abstract

Theoretical models predict that when having fat reserves is costly in terms of predation risk, birds should decrease their levels of fat reserves in response to increased predation risk. I performed an experiment in which yellowhammers were exposed to a control treatment, where a curtain was moved several times a day, 5 days in a row, and then to a predator treatment, where a perched, stuffed sparrowhawk appeared when the curtain was moved, 5 days in a row Between the two treatments were 2 days without any experimental treatment. The birds were expected to decrease in mass, and/or to change the daily trajectory of mass increase in response to increased predation risk. Yellowhammers decreased in morning mass and evening mass in response to both che moving curtain and the sparrowhawk compared to an untreated day before the start of the experiment. However, the response to both treatments was not the same; in the sparrowhawk treatment the birds waited longer before resuming feeding and lost more weight after each exposure as compared to the curtain treatment. This loss was regained, and yellowhammers increased their intake rate. Due to that, they reduced, although not significantly, the time spent feeding under predation risk. A reduction in the time spent feeding under predation risk reduces the time exposed to predators. However, if an increase in intake rate also incurs a decrease in vigilance, this might increase predation risk. The results of this study, together with other studies, indicate that for yellowhammers a reduction in time exposed to predators might be more important for survival than a reduction in body mass.