Aims: We evaluate sexual size dimorphism in Atlantic yellow-legged gulls Larus michahellis lusitanius and provide a discriminant function to sex gulls in hand.
Location: Two islands of the Basque Country (Northern Spain).
Methods: Incubating gulls were trapped, banded, weighted and measured. A drop of blood was extracted for molecular sexing. After testing for sex differences in body size and weight, discriminant function analyses were performed to identify the best traits for sexing.
Results: Body measurements in males were significantly larger than in females. Within each pair, males had larger head length, bill depth, long bill length and body mass. Discriminant analysis indicated than the combination of three measurements (head length, bill depth and wing length) predicted correctly the sex of 98.5 % of individuals.
Conclusions: The discriminant function described by Bosch (1996) cannot be used to identify sex properly in Atlantic Iberian yellow-legged gulls because they are significantly smaller than their Mediterranean counterparts. No significant differences were found with yellow-legged gull populations from Western extreme of Cantabrian coast (Galicia). Therefore, we conclude that the developed discriminant function can be applied to other Atlantic yellow-legged gull populations from Northern Iberian Peninsula, thus, providing a highly accurate, inexpensive and fast method for sexing in hand this Iberian gull subspecies.