Sea crossings of migratory pink-footed geese: seasonal effects of winds on flying and stopping behaviour

Migratory birds may need to cross barriers such as seas, without opportunities to rest or refuel. Waterbirds, unlike land birds, can stop at sea to rest or wait for better winds and thus may be less selective for supportive winds at departure and tolerate larger drift. However, pay-offs of waiting are likely to depend on circumstances (e.g. pressure for well-timed arrival, wind availability and travelling with/without juvenile brood), thus migratory behaviour during barrier crossings is expected to differ between seasons. We studied pink-footed geese Anser brachyrhynchus crossing the Barents Sea (ca 650 km), in spring and autumn during 2018–2020, using 94 GPS-tracks of 38 individuals, with annotated ERA5 weather data. We found that 1) especially in autumn, geese selected supportive winds for departure; 2) in spring, geese experienced lower wind support and more crosswinds than in autumn, leading to 23% longer routes, 60% longer durations, 93% longer air distances and 45% higher ratios of air-to-ground distances; 3) in both seasons, geese had more tailwinds in the first part of crossings, and in spring when deviating more from the shortest route; 4) geese stopped at sea more often in spring (mean 11×) than autumn (3×), in spring during earlier stages of crossings, but in both seasons, spent half of the crossing time at sea, during which they still continued to approach their destination slowly; 5) stops at sea happened mostly in adverse winds, warmer air, higher air humidity and on calmer water and, in autumn, took longer without juvenile brood. We conclude that for migrating pink-footed geese, Arctic capital breeders, the importance of time and energy can shift en route and that seasonal differences in wind support, flying and stopping behaviour and the pressure for a well-timed arrival cause the Barents Sea to be a larger barrier in spring than in autumn