Sex-specific growth analyses were conducted for 32 nestling ospreys (Pandion haliaetus) in a migratory population in Nova Scotia, Canada and for 31 nestling ospreys in a sedentary population in Sonora, Mexico. Eight variables including weight, body components and plumage characteristics were measured to document the influence of sex on growth performance. Within populations, males differed significantly from females in having lower weight and body component asymptotes but did not differ in plumage characteristics or growth rates. There was no difference in growth performance between individuals in broods of various sizes or within broods as a result of hatching order asynchrony. Comparisons of geographic variation showed that sedentary ospreys in Mexico had significantly higher weight and tarsus asymptotes, reduced growth rates, longer nestling periods and later emergence of flight feathers than migratory ospreys. Individual nestlings were initially identified by sex from karyotypic analysis of fibroblast tissue collected from a sample of 31 nestlings in the field. The karyotype is presented and growth performance is discussed within the framework of evolutionary theory.

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