Demography and dispersion: evaluating the causes and consequences of vagrancy in North American migratory birds

Rare, long-distance dispersal events in animals have long been of interest to both scientists and the public. However, the reasons for the occasional appearances of animals outside of their expected range, a phenomenon known as vagrancy, have largely lacked quantitative investigation. Demography is one hypothesized mechanism, where large and growing populations are thought to result in more individuals venturing farther afield. Alternatively, navigational mishaps brought on by less-than-ideal conditions may act to disperse individuals, ultimately resulting in vagrancy, especially for migratory species. We investigate the relative importance of demographic versus dispersive forces on vagrancy using over 4.5 million banding records across a half-century from 55 North American migratory bird species. We find evidence that both demographic and dispersive forces modulate vagrancy patterns over time, with population size explaining the most variation. Furthermore, for a subset of 42 species, we find no evidence that the dispersion of populations—a measure of the average spatio-temporal rarity of all banding records in a season—is associated with population declines. The combination of demographic and dispersive vagrancy could help explain the mechanisms behind range expansion of migratory animals and the organisms they disperse, with implications for biodiversity loss in a changing world.