Escape flights of yellowhammers and greenfinches: more than just physics

Wintering birds increase their fat reserves throughout the day, and impaire d escape performance is often considered to be an important cost of fat res erves. Since lifting a larger mass requires more energy, if birds escape at maximum power output, an increase in mass will impair the escape flight. I n this study we did not find support for mass-dependent escape performance for yellowhammers, Emberiza citrinella, and greenfinches, Carduelis chloris , with natural daily mass increases of 7-8%. This suggests either that the birds were not performing at maximum output at dawn, when light, or that ma ximum power output was higher at dusk, when heavy. Either way the birds see med to be able to put more effort into their escape flight when heavier. In both species, when alarmed, birds took off significantly faster and at a s teeper angle than when not alarmed. Yellowhammers escaped at a higher speed and angle than greenfinches, and reacted faster to the predator model. Thi s suggests that predator escape is more than just Newtonian physics, and ma y be influenced by behavioural, as well as morphological, adjustments. Diff erent species may have evolved different responses to predation risk. Our r esults seem to be in disagreement with recent ideas about mass-dependent pr edation risk. However, to build up reserves, birds have to increase exposur e time, which increases predation risk. This cost may be more important tha n impaired escape performance when relatively small, daily, changes in body mass are considered. (C) 2000 The Association for the Study of Animal Beha viour.