DIJK K VAN & R OOSTERHUIS (2008) Influence of catching method on observed sex ratio in breeding Black-headed Gulls Latus ridibundus on Griend (Wadden Sea). LIMOSA 81 (3): 94-97.

In 1999-2001 Black-headed Gulls were ringed in a large colony of around 25,000 breeding pairs on the island of Griend (53°15'N, 05°15'E) in the Dutch Wadden Sea. Their sexes were determined by a discriminant function based on structural measurements (Palomares et al. 1997) We observed remarkable differences in the sex ratio of trapped gulls between two catching methods used (Tab. 1). Birds trapped on the nest with a walk-in cage were males in 52.9% of 121 cases, agreeing well with data from the literature. In contrast, 80.7% of 270 adults (after second-calendar year) lured to a clap-net in the colony close to the wardens' with bread and small parts of fish were males. All catches were made between mid April and mid July, a period during which virtually all adults present on Griend are thought to belong to the local breeding population. We discuss that a biased sex ratio within the population seems unlikely, but differences between the sexes in feeding behaviour offer a good explanation. These may include differences in the choice of food and feeding location, and differences in dominance at the feeding location. Both have been suggested by previous studies, but future research is needed to clarify how exactly breeding male and female Black-headed Gulls differ in feeding behaviour.

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