Sexing adult and fledgling Lesser Black-backed Gulls from morphometrics

ABSTRACT Gulls are monomorphic in plumage characteristics but show a sexual size dimorphism where males are larger than females. In this study, we use discriminant function analyses (DFA) and generalised linear modelling (GLM) to provide sexing criteria based on morphometrics for adult and fledgling Lesser Black-backed Gulls Larus fuscus in Iceland. Males were significantly larger than females in all variables measured and this was the case for both age classes. For DFA the best sexing model for adults included skull length, bill depth and tarsus length and classified correctly 99.1% of the individuals sampled. For GLM only two variables, skull length and tarsus length, were needed for 100% classification of adult birds. The best sexing model for fledglings included skull length and tarsus length and classified correctly 89.4–91.5% of the individuals sampled, depending on the method. Individual sexing probability was much higher for adults than for fledglings, independent of the method used. However, individual sexing probability for adult birds was higher from applying GLM than from DFA. Higher success of GLM in assigning birds to the correct sex, higher individual sexing probability and more direct methodology to obtain a sexing probability make GLM an appropriate and recommended method for sexing birds.