Weights, Breeding, and Survival in European Sparrowhawks

Abstract

Abstract No significant diurnal variation in mean weight was detected in trapped European Sparrowhawks (Accipiter nisus). Weights of males fluctuated rather little through the year, but were highest in March and lowest in August. During the breeding cycle, males lost weight slightly in the prelay/early laying periods, when they were feeding females in preparation for egg production, and in the late nestling/postfledging periods, when they were feeding large young. Weights of females fluctuated greatly during the year: they were highest in May at the time of egg laying and lowest in August at the end of breeding. Females increased in weight by 40-50 g (15%) in the 10-20 days before laying; lost about 20 g over the laying period, on average; maintained high weight through much of incubation; and lost weight during the nestling period. The main functions of the extra body reserve are probably to buffer the female against temporary food shortages during incubation and to enable her to feed the chicks preferentially in the nestling period. Females that did not lay eggs remained low in weight throughout the early part of the breeding season and were then significantly lighter than breeders. Moreover, females that laid large clutches were heavier throughout the prelaying and laying periods than were females that laid small clutches, and females that subsequently hatched their eggs started incubation at a significantly greater weight than did those that deserted their eggs. We suggest that clutch size and breeding success are dependent on the female being able to maintain high weight during the periods she is dependent on the male for food and, hence, on the hunting success of the male. First-year birds of both sexes weighed less than adults throughout the year and were especially light during their first 2 months of independent life. This was probably a period of high mortality, especially in males, in which a clear relationship emerged between weight and survival prospects.