DIURNAL AND SEASONAL MASS VARIATION IN BLACKBIRDS TURDUS-MERULA - CONSEQUENCES FOR MASS-DEPENDENT PREDATION RISK

1. A bird's mass in winter should reflect the trade-off between the be nefits of an increased energy store to reduce starvation risk: and its costs in terms of increasing mass-dependent predation risk. State-dep endent models of this trade-off predict that as starvation risk increa ses then energy reserves should (i), increase, and (ii) be acquired ea rlier in the day. 2. Blackbirds increased their minimum weight by 25 /- 1.5% in midwinter; adults weighed significantly more than juveniles in midwinter, but weighed less in late winter. Weight for both sexes: and particularly adults, decreased with increasing daylength, but wei ght increases with respect to temperature decreases were most pronounc ed in males. Female blackbirds showed relatively small seasonal weight changes so that their midwinter mass-dependent predation risk relativ e to males was independent of diurnal weight changes. 3. Blackbirds lo st at least 1-9% of their body weight overnight depending on temperatu re. Over 60% of the weight lost was then regained in the first 3 hours after dawn, with only 5% being regained after 12.00 h. The pattern of early morning weight gain was maintained throughout the winter, with the ratio of weight gain rates in the first 4 h of dawn compared to th e rest of the day being c. 3:1 in midwinter (when individuals maintain ed high weights) and in early spring (when individuals were losing wei ght seasonally). 4. The results agree with the theoretical prediction that energy reserves should increase as risk of starvation increased ( as measured by seasonal factors such as cold, short midwinter days). T here was, however, little change in the seasonal pattern of diurnal we ight gain, with weight always being gained early in the day. In the li kely absence of foraging constraints within the study system, this res ult fits the predictions of state-dependent foraging models where anim als can escape from predation after achieving weight gains, by either using refuges or adopting low-risk foraging options.