Global patterns of migratory connectivity in barn swallows

Migratory connectivity defines the degree of mixing of individuals from multiple breeding populations in their non-breeding areas. It can vary across populations and may depend on migration distance and the availability of suitable land in non-breeding areas. Migratory spread — the average distance between individuals from the same breeding population in non-breeding areas — has been proposed as a measure of migratory connectivity. It has been hypothesised that migratory spread increases with migration distance due to increasing effects of random deviations from genetically predetermined routes, as well as with greater availability of suitable habitats. To test these hypotheses, we analysed a global dataset of nearly 300 geolocator-tracked barn swallows (Hirundo rustica ) from 12 populations, spanning from Argentina to eastern China. To our knowledge, this study is the first to examine migratory connectivity patterns within a single passerine species on a global scale. Barn swallows show broad variation in migratory behaviour, with some populations travelling short distances and others migrating long distances. Such variability was well captured by our global dataset, with migration distances ranging from 2559 to 8474 km and migratory spread from 675 to 2189 km across populations. Migratory spread increased significantly with greater availability of land masses in non-breeding areas but was unrelated to migration distance. Our results suggest that geography, rather than migration distance, drives intraspecific patterns of migratory spread. Historical and biogeographical factors, such as landmass distribution, likely shaped current connectivity patterns in this migratory species.