De groei van nestjongen van de Sperwer Accipiter nisus

The amount of food that is available to each chick in a brood depends on 1. food abundance, 2. the number of chicks in the nest and 3. the foraging efficiency by the parents. This paper aims to evaluate the importance of each of these factors in Sparrowhawk Accipiter nisus broods. Two habitats were discerned on the basis of forest type. Sparrowhawk population parameters show food-related differences between these populations in terms of density, first egg laying dates, and the proportion of yearling breeders. The condition of chicks was measured as the standardised residual of chick weight. To compare entire broods, average conditions were calculated per nest visit and per nest (two to six visits). Broods of three or four chicks had a significantly better condition than broods of five or six chicks. Variation in male performance at least equalled the difference between four and five chick broods and could compensate for habitat effects. Male performance has the strongest impact of all three factors above.