Sawada A., Akatani K. & Takagi M. (2021) Growth curves of Ryukyu Scops Owl nestlings, an owl species with asynchronous hatching and reversed sexual dimorphism. ARDEA 109 (2): 229-241

The growth pattern of an organism is influenced by multiple factors, such as its sex, parental investment, food abundance and sibling competition. For species showing sexual size dimorphism, the sex of siblings is expected to affect the growth of nestlings through differences in competitive hierarchies mediated by an asymmetry in body size. This study describes growth curves of Ryukyu Scops Owl Otus elegans nestlings, based on the analysis of body mass, wing length and tarsus length in 99 individuals. We subsequently tested for effects of sex of the nestlings, having an older male sibling, hatching order and brood size on the growth curves. Overall, we detected significant effects of several predictor variables, although differences between groups were often small. Males and later hatched chicks had lower body mass throughout their growth. The smaller the brood, the heavier the chicks were on days 0–1 and 30–32, although this relationship was reversed on days 8–13. Males and later hatched chicks also had shorter wings. Males had shorter tarsi than females from day 0 to day 4, however, dimorphism in this trait was reversed afterwards. Chicks in a large brood had longer tarsi. Earlier hatched chicks had longer tarsi in the first half of the growth period, but some of this difference had disappeared by the second half. From day 0 to day 3, individuals with an older brother had longer tarsi than those with an older sister. However, halfway through the growth period and thereafter, individuals with an older brother had shorter tarsi than those with an older sister. The fact that older brothers had an effect on the tarsus growth of their siblings was not due to sexual body size asymmetry, as in fact males were smaller than females in wing length and body size. We suggest that sexual differences in aggression mediated by androgens may have played a role.