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 sea..., Between mid-May and early July 2016 – 2020, we caught 84 incubating common terns a total of 229 times with a drop trap, on average 16 ± 4.3 SD days after the first egg in their nest was laid. These birds were then equipped with a geolocator attached to the leg using a 10 mm aluminium ring [Kürten et al. 2019]. The geolocators were set to sample ambient light intensity every minute, with the maximum light intensity being stored every five minutes (mode 10). The total mass of the geolocator, ring and glue was 1.6 g, which equalled 1.2% ± 0.1 SD of the body mass of the birds at capture (mean body mass: 129.1 g ± 8.1 SD). In the breeding seasons of 2017 – 2021, we re-trapped the 84 individuals equipped with a geolocator that returned (i.e. 86%) a total of 198 times, and recovered 192 devices with light-level data (i.e. 97%). As such, we obtained data for 192 post-breeding migratory journeys performed by 84 individual birds (41 males and 43 females), as well as 149 pre-breeding migratory jou..., , # Route flexibility is associated with headwind minimization in a long-distance migratory seabird

[https://doi.org/10.5061/dryad.2z34tmpwr](https://doi.org/10.5061/dryad.2z34tmpwr)

## Description of the data and file structure

Daily position data for our multi-year individual-based tracking study were collected in a common tern colony located at the Banter See in Wilhelmshaven at the German North Sea coast (53°30’40” N, 08°06’20” E). The data were used to test (i) whether birds show within-individual spatial similarity in the migration routes taken across years, and (i) to assess whether birds leaving within the same time window (and hence experiencing similar environmental conditions) take similar migration routes, and, if so, (iii) whether such among-individual temporal route similarity could be associated with minimisation of the encounter of unfavourable wind conditions.

Contact Nathalie Kürten ([[e-mail]](mailto:[e-mail])) with any questions. This...