The effect of predator presence on body mass in captive greenfinches

Many passerines wintering in temperate regions rely on their energy reserves for survival during periods when foraging is not possible. Excessive fat reserves, however, increase body mass and thus reduce agility and speed in flight, imposing a survival cost when the bird tries to escape from predators. If fat reserves reflect a trade-off between starvation and predation risks, the size of reserves could be expected to vary with the perceived level of predation risk. The hypothesis that increasing the relative risk of predation results in decreased body mass was tested experimentally using captive greenfinches, Carduelis chloris. Greenfinches show winter fattening and take flight when disturbed and are therefore suitable for testing the hypothesis. The perceived predation risk, a stuffed predator moved in the birds' environment, resulted in lower body mass. Handling the birds had the same effect. These results support theoretical predictions and some field data regarding the adjustment of body mass to variation in the predation risk. They further suggest, in line with other studies, that birds are able to change body mass in response to proximate factors. (C) 1997 The Association for the Study of Animal Behaviour.