Breeding Phenology and Sibling Competition Drive Nestling Growth in Eurasian Eagle-Owls Bubo bubo

Appropriate growth of a nestling determines its subsequent reproductive ability. Numerous studies demostrate the influence of various factors on the growth rate but only a few deal with nocturnal species. Furthermore, no analyses have been carried out for this species group to examine three parameters of the growth function, i.e., the expected final size, the start of rapid growth, and the growth rate constant. We analysed the influence of breeding season timing and a likely correlate of sibling competition on the growth rate of Eurasian Eagle-owl Bubo bubo nestlings. We found that the rapid mass growth phase started earlier and reached its final value earlier than for wing length. Furthermore, wing length growth over the season took longer in nests with more siblings but ultimately reached greater length than in the smaller broods. We also found that as the season progressed mass growth took longer and resulted in smaller final size than in individuals that started breeding earlier.