Coal tits, Parus ater, lose weight in response to chases by predators

Theoretical models predict that birds should decrease their body mass in re sponse to increased predation risk because lighter birds take off faster an d are more manoeuvrable. We studied the effect of predation risk by chasing coal tits in large outdoor aviaries thus simulating an attempt to capture them. With this increase in predation risk, both perceived and actual, coal tits lost significantly more weight than in a control situation when they were not pursued. This pattern was attributable to a smaller gain in weight only during the day; nocturnal weight did not change in relation to diurna l predation risk. The lower daily weight gain was not consistent with predi ctions from models of interrupted foraging, but war consistent with predict ions from risk adjustment models. Moreover, there was no difference in weig ht gain over 2-h periods that included a 1-h fast and those in which feedin g was ad libitum, suggesting that coal tits could easily regain their body mass after a predator had interrupted their feeding. Our results therefore suggest that pursuit by predators leads to a decrease in the body mass of s mall birds. (C) 1999 The Association for the Study of Animal Behaviour.