IMPAIRED PREDATOR EVASION IN FAT BLACKCAPS (SYLVIA-ATRICAPILLA)

When birds are attacked by predators, take-off ability is crucial for the chance of survival. Recently, theoretical studies have predicted t hat predation risk in terms of reduced flight performance increases wi th body mass. However, empirical data are scarce. Because migratory bi rds sometimes double their body mass, mass dependent predation risk ma y be especially important during migratory fattening. Here we present the first study of take-off ability in relation to migratory fat load. Alarmed take-off flights of caged blackcaps (Sylvia atricapilla) indu ced by a simulated predator attack were analysed in terms of velocity and angle of ascent. Fat loads (percentage of fat-free body mass) of t he birds ranged from 1% to 59%. An increase in fat load was found to i nfluence both velocity and angle of ascent. From our results we calcul ated that blackcaps carrying 60% fat loads would have 32% lower angle of ascent and 17% lower velocity than blackcaps carrying no fat load. Even though the effect of fat load on the blackcaps was less than indi cated in previous experimental studies of other species, our results s uggest that the large fat loads needed for migration probably place th em at increased risk of predation.