Route flexibility is associated with headwind minimization in a long-distance migratory seabird

Seasonal migration has evolved across taxa and encompasses a multitude of features, many of which vary between species, between and within populations, and even within individuals. One feature of migration that appears especially variable within individuals is the route taken to reach a destination, even when the destination itself is not variable at this level. To investigate why, we analysed the geolocator tracks describing 192 post-breeding migratory journeys of 84 common terns (Sterna hirundo), as well as 149 pre-breeding migratory journeys of 75 of these birds. We found little within-individual spatial consistency in migration routes across years, irrespective of season or sex. Instead, individuals departing during the same time window took similar migration routes, which, during pre-breeding migration, when birds predominantly encountered headwinds, were associated with minimized headwind exposure. We therefore suggest that the individual routes of this long-distance migratory seabird can be flexibly adjusted to environmental variation, which is likely to be adaptive.