Węgrzyn E. (2012) In the Blackcap Sylvia atricapilla last-hatched nestlings can catch up with older siblings. ARDEA 100 (2): 179-186

Development during the relatively short time that young spend in the nest is crucial in a bird’s life history. The rate of nestling growth strongly depends on numerous environmental and social factors, even within the same species or population. In the present study I examined the relative contributions of brood size, hatching asynchrony, hatching date and feeding rate in determining intraspecific variation in the growth of Eurasian Blackcap Sylvia atricapilla nestlings. I found a significant effect of brood size, hatching order and date on nestling development. However, the effect of hatching order and date was significant only for young nestlings and it gradually disappeared with nestling age. The only factor significantly affecting nestling development just before fledging was brood size. Nests of four siblings produced the heaviest young with the longest tarsi. This probably resulted from the highest feeding rate per nestling observed in broods of four nestlings. The study shows that for Blackcaps breeding in a good year a brood size of four is the optimal number, and that younger siblings can catch-up with older nest mates.