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 minimised 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.