Sexing Bonelli's Eagle nestlings: Morphometrics versus molecular techniques

We report details of two PCR-based molecular sexing techniques for the Bori elli's Eagle (Hieraaetus fasciatus) and evaluate the reliability of morphom etric measurements to predict the sex of nestlings in the field. Blood samp les taken from 63 nestlings in southwest Portugal (1994-99) were analyzed u sing the intron polymorphism method (MI), and 56 of these were also analyze d with the single-strand conformation polymorphism approach (M2). Contamina tion or poor preservation of samples precluded one sex determination with M 1 and six others with M2. Sexing by both methods was concordant for 98.0% o f samples. Linear discriminant analysis was used to determine whether any s ingle variable or combination could provide reliable sex determinations, us ing 10 body measurements from 43 nestlings aged 35-50 d, sexed unambiguousl y by both molecular methods. Models were evaluated by cross-validation of t he original data and from the classification of an external sample (N=12). Females were significantly larger than males. The greatest separation betwe en sexes occurred in body mass, but differences were also noted in tarsus d iameter and the lengths of the hind claw, foot, culmen, and forewing; no di fferences were detected in the lengths of tarsus, fore claw, seventh primar y, and central tail feather, A discriminant model including body mass, hind claw length, and age provided the maximum separation between sexes and it correctly sexed 96% of the nestlings. A model including tarsus diameter, hi nd claw, and age showed similar accuracy. Both models were satisfactory in determining the sex of nestling Bonelli's Eagles between the ages of 35-50 d in the field, but combination with molecular techniques may be preferable in studies requiring absolute precision for every individual handled.