Immune function and infection status is related to migration distance and phases of the annual cycle in song sparrows (Melospiza melodia)

Life history theory predicts animals adjust immune investment based on their risk of encountering familiar and unfamiliar parasites. Although long-distance migrants may encounter a greater diversity of parasites than short-distance migrants, the energetic costs of migration may constrain immune investment. However, few studies have investigated the relationship between migration distance and immunity. I used stable isotope analysis to estimate the wintering latitude, and thus migration distance, of song sparrows (Melospiza melodia). I assessed whether migration distance was related to the incidence of infection with a blood-borne parasite, parasite load, and immune function. The incidence of infection increased with migration distance, but only for adults. Macrophage phagocytosis, associated with innate immunity, decreased with migration distance but only for males. As birds shifted from migratory to breeding condition, innate immunity decreased while acquired immunity tended to increase. These findings suggest that exposure to parasites varies with migration distance, the demands of migration may constrain immune investment, and innate immunity may be important during migration