We studied the influence of host age and sex on the helminth fauna of 324 Larus michahellis captured in different locations in the region of Galicia (northwestern Spain). Gulls were grouped into prefledglings, first-year immature birds, second- and third-year immature birds, and adults. Second-year, third-year, and adult birds were grouped by sex. Thirty-six helminth species were recorded. Total species richness and mean infracommunity species richness were both significantly lower for prefledglings than for the other age groups. Prevalence increased significantly with age for Brachylecithum microtesticulatum, probably reflecting changing feeding habits. Likewise, 8 species (Cardiocephaloides longicollis, Microphallus similis, Maritrema gratiosum, Gynaecotyla longiintestinata, Brachylecithum microtesticulatum, Himasthla elongata, Parorchis acanthus, and Renicola sp.) were absent or had very low prevalence in prefledglings. At least 5 of these 8 species are transmitted to gulls through ingestion of molluscs or crustaceans, which suggests that these types of prey are seldom fed to prefledglings. In Gymnophallus deliciosus, G. longiintestinata, and Cosmocephalus obvelatus, mean intensity, and in the latter case prevalence, declined with age, suggesting that protective immunity against these species increase with age. Only G. deliciosus, Microphallus similis, and G. longiintestinata presented significant differences between the sexes.

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