A helminth community in breeding Yellow-legged Gulls (Larus cachinnans): pattern of association and its effect on host fitness

We studied the community of helminths parasitizing adult Yellow-legged Gull s (Larus cachinnans) sampled during three consecutive breeding seasons in a western Mediterranean colony. The helminth community was depauperate, prob ably because of the high trophic dependence of the gullery on waste food fr om refuse dumps. The prevalences of two helminth species varied both among years and between the sexes of gulls, while the intensity of parasitism did not vary significantly for any helminth species. No clear pattern of assoc iation between helminth species was detected, showing an unstructured helmi nth community in which the occurrence of any species was not restricted or favoured by the occurrence of any other species. The lack of associations m ight be influenced by the fact that only one core species occurred in the h elminth community. Negative relationships were found between occurrence of certain helminths and body condition of hosts. Gulls infected by Tetrabothr ius (Tetrabothrius) erostris were leaner than uninfected gulls; interesting ly this negative relationship varied with the presence/absence of Cosmoceph alus obvelatus. Moreover, body condition of gulls infected by C. obvelatus varied according to the year of sampling, while that of uninfected individu als showed no variability. High intensities of the parasites Eucoleus conto rtus and Brachylaima sp. were associated with poorer body condition of the host. In the case of E. contortus, this negative relationship with gulls' b ody condition was more pronounced in male than in female gulls. The results show that helminth pathogenicity can be affected by helminth community str ucture, environmental factors, and traits of the host.