Effects of fat reserves on annual apparent survival of blackbirds Turdus merula

Abstract

Summary Fat reserves are stored energy that may help birds survive periods of harsh winter weather. This hypothesis predicts that annual apparent survival is higher for birds with large fat reserves than for birds with few or no fat reserves in winter. Blackbirds (Turdus merula Linnaeus) were ringed in central Italy from 16 November to 20 February during 1990–2001. Fat scores were recorded for each bird. We used these capture–mark–recapture data for 1703 blackbirds to estimate the effect of large fat reserves on annual apparent survival, while controlling for transients, using computer programs surviv and mark. Probability of birds retaining large fat reserves, or retaining few fat reserves, over 2 successive years was also estimated. Birds with large fat reserves did not have higher estimated annual apparent survival than birds with few fat reserves (φ̂large= φ̂few= 0·595, SE = 0·043), inconsistent with our prediction. No effects of age, sex or year were detected on annual apparent survival. Birds with few fat reserves in any given year tended to have few fat reserves the following year (, SE = 0·052). Birds with large fat reserves in any given year were unlikely to have large fat reserves the next year (, SE = 0·080). Large fat reserves may not increase annual survival of blackbirds wintering in central Italy. Winter weather in our study area may be too mild to effect survival. Alternatively, increased predation risk associated with large fat reserves may counteract any benefits of reduced starvation risk.