Parasitism, immunity, and arrival date in a migratory bird, the barn swallow

Long-distance bird migration is expensive, and individuals in prime condition are therefore expected to arrive at the breeding grounds earlier than the average individual in the population. We tested whether arrival date during spring migration was associated with low parasite burden and strong T-cell-dependent immune response in populations of the transequatorially migrating Barn Swallow Hirundo rustica from Denmark, Italy, and Spain. Male Barn Swallows with heavy infestations of the chewing louse Hirundoecus malleus L. arrived later than other males, whereas that was not the case for females. This negative relationship remained after controlling the relationship between parasite load and arrival for tail length. Infection with blood parasites of the species Haemoproteus prognei was associated with delayed arrival in the Spanish population, which was the only one with a high prevalence of blood parasites. High intensities of infestation with a commensal feather mite were associated with early arrival by females, but also by males when the independent relationship between tail length and intensity of mite infestation was controlled statistically. Experimental manipulation of nest infestation with the hematophagous mite Ornithonyssus bursa Berlese affected the arrival date of adult males, but not of females in the following year, with arrival date being inversely related to the experimental mite population size. Males with strong T-cell responses arrived earlier than males with weak responses, independent of their tail length in two of three populations, but there was no significant relationship in females. These findings are in accordance with predictions based on the hypothesis that bird migration is dependent on condition, with individuals in prime condition arriving early.