Discriminant functions based on external body size measurements are widely used to sex different gull species with great accuracy. However, all of them have been derived for adult birds, which puts into question their usefulness for sexing immatures due to possible changes in size as birds mature. To address this issue, discriminant functions that allow sexing of Herring Gulls in immature age classes with an accuracy of 88-100% were developed. In total, 247 males and 111 females of wintering Herring Gulls, including birds in the first, second and third winter plumages and individuals in adult plumage, were measured and sexed in the region of the Gulf of Gdansk (southeastern Baltic coast). In all age classes, total head length and bill depth were the best traits for sexing Herring Gulls. However, bill depth, but not total head length, increased with age. Hence, in the first and second winter plumages, total head length made a much higher contribution to the discriminant function than bill depth. In the third winter plumage, bill depth became more important. For individuals in adult plumage, however, the contribution of total head length and bill depth were nearly the same. Hence, using discriminant equations derived for adults resulted in erroneous sexing of 4.5-8.9% of immature males, which were identified as females, and illustrates the importance of deriving age-specific discriminant functions. [ABSTRACT FROM AUTHOR]