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 benefits of an increased energy store to reduce starvation risk, and its costs in terms of increasing mass-dependent predation risk. State-dependent models of this trade-off predict that as starvation risk increases then energy reserves should (i), increase, and (ii) be acquired earlier 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 weight increases with respect to temperature decreases were most pronounced in males. Female blackbirds showed relatively small seasonal weight changes so that their midwinter mass-dependent predation risk relative to males was independent of diurnal weight changes. 3. Blackbirds lost at least 1-9% of their body weight overnight depending on temperature. 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 4h of dawn compared to the rest of the day being c. 3:1 in midwinter (when individuals maintained high weights) and in early spring (when individuals were losing weight 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). There was, however, little change in the seasonal pattern of diurnal weight gain, with weight always being gained early in the day. In the likely absence of foraging constraints within the study system, this result fits the predictions of state-dependent foraging models where animals can escape from predation after achieving weight gains, by either using refuges or adopting low-risk foraging options.