Nestling sexing by biometric and molecular analysis in two diurnal raptors, Circaetus gallicus and Buteo buteo.
Comparison with the Goshawk (Accipiter gentilis) and adult Buzzards

Abstract. The following data were collected during a long term study in France of the Common Buzzard Buteo buteo and the Short-toed Eagle Circaetus gallicus. Even though females of these two species are significantly larger than males, biometric parameters show substantial overlaps between sexes, so that in-hand sexing from morphometrics alone can be hazardous. Moreover, plumage differences are unavailable for juvenile birds, and both sexes are identical at hatching. Therefore, we have been looking for a reliable method of sexing nestlings, based on discriminant analysis of measurements, computed between groups of DNA determined males and females. In this paper, we give simple (linear) functions of a few parameters, allowing the prediction of the sex of a good proportion of the juveniles:

Roughly 85% of Short-toed Eagle nestlings, older than 40 days (wing-length > 170 mm), and 90% of Common Buzzard nestlings, from the age of 4 weeks (wing-length > 205 mm), can be sexed correctly, using:

• tarsus diameters for Circaetus gallicus

• tarsus diameters and bill length for Buteo buteo.

In comparison, the same methods can predict correctly the sex of more than 90% of adult Common Buzzards (using bill-depth, wing-length, and body mass), and discriminate all males from female Goshawk nestlings Accipiter gentilis, (using tarsus diameters and weight), provided that their wings are longer than 180 mm.

Thus, tarsus diameters and bill length are the most accurate parameters to determine the sex of fledglings. However, the overlaps between measurements are so substantial that we have not found any function which could perfectly discriminate males from females in Buteo buteo and Circaetus gallicus. An analysis of the length of hallux claw, forearm and footpad could provide a higher reliability, but probably would not reach perfection yet.

A first application of our study shows that, out of 207 buzzards and 302 Short-toed Eagles, the sex ratio of juveniles is approximately 1:1 for the eagle and 60 females: 40 males among buzzards, in our study areas.

Mots clés: Buteo buteo, Circaetus gallicus, Poussins, Analyse discriminante, Biométrie, Détermination du sexe par analyse moléculaire.

Key words: Buteo buteo, Circaetus gallicus, Nestlings, Discriminant analysis, Biometrics, Molecular sexing.