DOES DIURNAL-VARIATION IN BODY-MASS AFFECT TAKE-OFF ABILITY IN WINTERING WILLOW TITS

Recently, some empirical studies have shown an effect of body mass on take-off ability in birds, supporting the theory that birds with incre ased fat loads face a greater predation risk. Small wintering birds ac cumulate fat reserves of about 10% of their fat-free body mass during the day to survive long, cold, winter nights. Theoretically, mass-depe ndent predation risk could be an important factor affecting their dail y foraging routines. I studied the take-off ability of wintering willo w tits, Pants montanus, in relation to their fat load. Willow tits wer e on average 7.7% heavier at dusk than at dawn but there was no measur able effect of body mass on take-off ability. The results indicate tha t the relatively small fat loads accumulated during a day by willow ti ts do not increase the risk of predation as a consequence of reduced t ake-off ability. (C) 1998 The Association for the Study of Animal Beha viour.