Sex-Specific Growth in Ospreys: The Role of Sexual Size Dimorphism

Abstract

Sex-specific growth analyses were conducted for 20 male and 12 female Ospreys (Pandion haliaetus) in 19 broods in Nova Scotia, Canada during 1984-1986. The graphical methods of Ricklefs showed the logistic model to be most appropriate for mass increase with a growth-rate constant K of 0.18 days-' for both sexes combined. The influence of sex on growth performance was also described using a reparameterized Richards model. Mass and tarsus length were the only variables to show well-defined asymptotes at fledging, although talon length, cranium width, and culmen length were within 10% of adult values. Males differed significantly from females in having lower asymptotes of mass and tarsus length, but did not differ in rate of growth. There was no difference in growth rates between individuals in broods of one, two or three nestlings, or within broods as a result of hatching- order asynchrony. Males and females showed no difference in age at time of feather emergence or in length of nestling period. Thus, because there was no evidence for rapid growth in males in order to compete with larger females for food resources at nests, we suggest that sexual size dimorphism may be independent of sibling competition in Ospreys. Received 8 FISHER'S (1930) sex-ratio theory is based on the assumption that natural selection should favor parents who invest equally in male and female offspring, resulting in a population sex ratio of unity or 50:50. To test Fisher's theory, workers have turned to investigating sexually dimorphic species where sex ratios other than 1:1 can be predicted. To date, however, studies with sufficient sample size have discovered lit- tle evidence of sex-ratio deviation in either rap-