Genetic and morphological sex identification methods reveal a male-biased sex ratio in the Ivory Gull Pagophila eburnea

Sex identification of birds is relevant to studies of evolutionary biology and ecology and is often a central issue for the management and conservation of populations. The Ivory Gull Pagophila eburnea (Phipps, 1774) is a rare high-Arctic species whose main habitat is sea ice throughout the year. This species is currently listed Near Threatened by the IUCN, because populations have drastically declined in part of the species distribution in the recent past. Here we tested molecular sexing methods with different types of samples. Molecular sexing appeared to be very efficient with DNA extracted from muscle, blood, and buccal swabs, both for adults and young chicks. We also performed morphological analyses to characterize sexual size dimorphism in Ivory Gulls sampled in three distinct regions: Greenland, Svalbard, and Russia. Males were larger than females for all morphometric measurements, with little overlap between sexes. Discriminant analysis based on six morphometric variables correctly classified ~95 % of the individuals, even when using two variables only, i.e., gonys height and skull length. Therefore, both molecular and biometric methods are useful for sexing Ivory Gulls. Interestingly, our results indicate a male-biased sex ratio across all Ivory Gull populations studied, including two samples of offspring (67.8 % males).