Sex- and age-related biometric variation of Black-headed Gulls Larus ridibundus in Western European populations

Abstract

A multivariate discriminant analysis was performed with biometric data from 583 Black-headed Gulls Larus ridibundus of known sex, either wintering in central Spain or wintering and breeding in The Netherlands. The discriminant function calculated from these birds was applied to 1158 Black-headed Gulls trapped in Madrid; age, sex and temporal differences in biometrics were analysed for those birds. All measurements taken varied significantly between the sexes, but head-and-bill length and bill depth were the best discriminant variables. Bone measurements (bill length, head-and-bill length, and tarsus length) did not vary between first- or second-winter birds or adults. In contrast, wing length, length of the eighth primary, mass and bill depth increased significantly with age for both sexes, although females attained their final size quicker than males. Both wing length and body condition increased throughout the winter. Possible explanations for these results are discussed.