NESTLING WEIGHT AND JUVENILE SURVIVAL IN THE BLACKBIRD, TURDUS-MERULA

(1) This paper describes the survival of juvenile blackbirds according to their weight as 8-day-old nestlings. In a 4-year study of 748 individually colour-ringed nestlings, heavy nestlings were more likely to survive to breed in the study area than lighter nestlings. (2) Nestlings below 35 g never survived to fledge; those above 45 g showed a linear increase in the probability of survival to independece, up to heaviest nestlings in the sample; between 35 and 45 g there was a linear, but more rapid, increase in survival prospects with weight. The shape of the relationship between nestling weight and juvenile survival was similar in different years, through the season, and between brood sizes and age ranks within broods. (3) Differential mortality by nestling weight occurred from ringing to fledging, from fledging to 2 weeks after fledging (when the young are still fed by their parents and are easily located on territories), and from 2 weeks to 1 month after fledging (when the young are independent but can be difficult to locate). However, birds that survived this period apparently had a random chance of breeding. (4) These findings on juvenile survival suggest that nestling weight itself influences juvenile survival, not some correlate such as brood size. (5) A knowledge of the shape of the relationship between nestling weight and juvenile survival allows one to measure nestling 'quality' as the probability that a nestling will survive to breed. In practice, the probability of survival to shortly after nutritional independence might be an adequate estimate of relative probabilities of recruitment to the breeding population in many bird species.