Seasonal migration distance varies with natal dispersal and predicts parasitic infection in song sparrows

Abstract

Seasonal migration and natal dispersal represent the major large-scale movements in the lives of animals. Individuals that are relatively prone to movement and exploration might thus be more likely to disperse and also to migrate farther. Such movement might be either negatively associated with parasitic infection (if infection prevents hosts from successful long-distance migration) or positively associated (e.g. if longer-distance migrants encounter more abundant or more diverse parasites). We examined whether natal dispersal tendency predicts seasonal migration distance in song sparrows (Melospiza melodia) and whether migration distance predicts infection with bloodborne parasites upon arrival at the breeding grounds. Migration distance, inferred from stable hydrogen isotope analysis (δ²H) of winter-grown tissue, was repeatable (repeatability = 0.41) over years. Birds that were more likely to have immigrated from outside the breeding grounds, as inferred from genetic assignment tests, also overwintered farther south, as inferred from stable isotope analysis. The finding that individuals more prone to movement in the context of natal dispersal also tended to travel farther, on average, in the context of seasonal migration suggests consistent individual variation in large-scale movements across these two contexts. Although statistically significant, this effect was modest in scope and subtle relative to sex differences in inferred migration distance. Among after-second-year individuals, but not yearlings, longer-distance migrants were more likely, on average, to be infected with bloodborne parasites. Individual variation in propensity to long-distance movement may interact with age-related variation in exposure or susceptibility to parasites, to shape the role of animal migration in transporting infectious disease.

Significance statement

In many animal species, individuals vary consistently in activity and movement over small geographic scales. Large-scale movements such as seasonal migration are of particular interest because they can influence disease risk, but until recently, such movements have been difficult to track. We used stable isotope analysis to estimate overwintering latitude of song sparrows returning to their breeding grounds. Individuals overwintering farther south were also more likely to have immigrated from outside the breeding site, indicating that movement is correlated across the contexts of migration and dispersal. Rates of infection with bloodborne parasites increased with migration distance in older birds, but not in yearlings. Our findings suggest that individual variation in movement may interact with age-related variation in infection risk, to shape the role of animal migration in transporting parasites.