In this thesis I show that captive yellowhammers (Emberiza citrinella) strategically adjusted their level of body mass and diurnal trajectory of mass increase to experimentally increased predation risk. During winter, small birds need large fat reserves to survive the night. Therefore, they build up fat reserves every day. However, having fat reserves, i.e. being heavy, may be costly in terms of predation risk. By decreasing their level of body mass and postponing their mass increase to the end of the day, the birds decreased those costs, and, thus, decreased the chance of being preyed upon. Through these strategic processes, the birds did not only decrease their predation risk, but also increased their starvation risk, resulting in another trade-off between starvation and predation risk.
One of the proposed costs of fat reserves is impaired escape performance. However, in this thesis I show that, for diurnal changes in mass, there was no impaired escape performance. Yellowhammers seem to be able to compensate for their larger mass at dusk with higher energy expenditure, resulting in the same speed and angle at dawn and dusk.
A predator encounter has a striking effect on the body mass of yellowhammers. Upon each exposure, the birds lost a considerable amount of mass. This mass loss was not as much due to the feeding interruption the birds experienced upon exposure, but was to a large extent due to predator stress. If time for foraging is short, this mass loss may put constraints on the level of body mass. Such involuntary losses may mask strategic adjustments in mass to increased predation risk.
In the field, yellowhammers did not postpone foraging to the end of the day. Instead, they concentrated their foraging in the morning. In contrast to my experiments, predators were not evenly divided over the day, but were more frequent after noon. Yellowhammers may have been avoiding predation risk by foraging early in the day. This effect disappeared when birds were under time-stress. These results indicate that the outcome of the trade-off will differ depending on the environment and the state of the individual.
Uppsala: Acta Universitatis Upsaliensis , 1999. , 33 p.
1999-11-26, Elias Fries-salen at the Evolutionary Biology Center, Uppsala University, Uppsala, 10:00