Reliability of morphological criteria for sexing birds during ringing, assessed using molecular methods – a study of thirteen species of passerines and near passerines

ABSTRACT Understanding the ecology and conservation of bird species often requires accurate sex determination of individuals. Species with sexually dimorphic plumage can usually be sexed in the hand based on consistent and definitive differences in plumage between sexes, but there are often challenges related to (1) how sexual dimorphism develops with age, (2) individuals that show intermediate visible morphological traits, or (3) consistent but subtle trait differences that require considerable experience to use reliably. Species with sexually monomorphic plumage, which constitute over half of all avian species globally, pose a greater challenge and can often not be sexed in the hand. The aim of this study was to use molecular methods to identify definitively the sex of individuals of both monomorphic and dimorphic species caught at a ringing site in south-west Portugal, in order to evaluate the standard morphological sexing techniques for species showing sexual dimorphism in plumage, or in biometric measurements. Blood samples were collected from a range of species during ringing, and DNA was extracted. Molecular methods were successful in identifying the sex of 202 individuals across 13 species of birds (eight species with sexually dimorphic plumage, and five sexually monomorphic in plumage). Molecular methods were consistent with the morphological sexing in the field for six of the eight species with dimorphic plumage, but discrepancies between the two methods were identified for Pied Flycatcher Ficedula hypoleuca and Eurasian Hoopoe Upupa epops. Finally, biometric measurements taken in the field were used to assess whether species with monomorphic plumage could have been correctly sexed based on the biometric differences between males and females reported in literature.