Intracolonial differences in the infestation by Haemoproteus lari on Yellow-legged Gulls Larus cachinnans

The prevalence and intensity of Haemoproteus IaH infestation were studied in a colony of Yellow-legged Gulls (Larus cachinnans) on the Medes Islands (NE Spain) during 1994 and 1995. Prevalence did not differ between years, sexes, clutch sizes or nesting habitats. More heavily infected females tended to lay smaller clutches and to be in a leaner body condition; in males, intensity of parasites was unrelated to their clutch size and their body condition. These results are partially consistent with the detrimental effect of blood parasites, but also with a tendency for stressed birds to relapse. Intensity of parasitemia differed both between years and among nesting habitats of the gullery, probably as a result of differences in bird quality at the time of sampling. We suggest that these effects should be controlled for in studies dealing with parasites in colonial nesting birds.