Body mass variations in disturbed mallards Anas platyrhynchos fit to the mass-dependent starvation-predation risk trade-off

For passerines the starvation-predation risk theory predicts that birds should decrease their body mass to improve escape flight performance, when predation pressure increases. To investigate whether this theory may apply to large birds, which manage body reserves differently from small passerines, we experimentally increased the predation risk in mallards Anas platyrhynchos. Two groups were disturbed at different frequencies during experimental sessions lasting one week, while a control group was left undisturbed. We found that body mass loss and final wing loading were similar in both disturbed groups and significantly differed from the control group. Food intake in disturbed groups was reduced up to day four of the disturbance session and was lower than in the control group. Altogether our results suggest that disturbed mallards may adjust their body mass to reach a more favorable wing loading, supposedly to improve escape flight performance. Nevertheless, body mass loss in our mallards was double than what has been observed in passerines. This greater mass decrease might be explained by different strategies concerning energy storage. Furthermore, in large birds the predation component of the starvation-predation trade-off might be of greater importance. Hence, the observed relevance of this trade-off over a large size range suggests that the starvation-predation risk theory is of major ecological significance for many animal species.