Comparative analysis reveals a possible immunity-related absence of blood parasites in Common Gulls (Larus canus) and Black-headed Gulls (Chroicocephalus ridibundus)

Blood parasites often incur a substantial fitness cost to the infected individuals, sometimes resulting in death of the host. Some bird species, however, are apparently free of blood parasites, presumably due to the lack of exposure to blood parasite vectors. Protective immunity may be also responsible for the absence of infections by haematozoa. In this study, we tested the presence of blood parasites in Common Gulls (Larus canus) and Black-headed Gulls (Chroicocephalus ridibundus) nesting in environments with varying vector exposure. We failed to find blood parasites in Common Gulls irrespective of vector exposure, whereas infection rates of Black-headed Gulls were generally very low. We propose that the absence of haematozoa and low prevalence of blood parasites in these species of gulls is probably not a function of vector exposure and suggest alternative explanations such as enhanced immunity.