Seasonal variation in trans-oceanic flight and island use in a migratory falcon.

Wind conditions play a major role in determining how birds negotiate ecological barriers during long-distance migration. Migrants typically minimize flights over barriers in opposing winds, while crossing the same barriers directly in supportive winds. Nevertheless, even in supportive conditions, direct barrier-crossings may be associated with an elevated risk of fatigue. At the same time, barrier-crossings may be facilitated by the availability of steppingstones. We aimed to understand how seasonal ocean-crossings of Eleonora's falcons (Falco eleonorae) compare in terms of wind support, flight effort, and the use of remote islands. To do this we combined over a decade of GPS-tracking data from 19 individuals (2012 to 2022) with wind data from an atmospheric reanalysis model. Despite far greater wind support in spring, falcons achieved similar ground speeds in both seasons, because they flew at much lower airspeeds in spring (8.8 ± 1.7 m/s) than in autumn (13.2 ± 2.6 m/s). Overall, it took falcons twice as many flight hours to complete a 40% greater air distance in spring compared with autumn. More islands are available along the spring route, and 57.9% of falcons used an island during at least 1 spring crossing, compared with 21.1% in autumn. In spring, island use increased with weaker wind support, with falcons spending the night on islands, and apparently maximizing daytime flight by initiating ocean-crossings at dawn. Overall, wind conditions only partially offset the seasonal difference in falcons' crossing distance and effort. Isolated islands may be an overlooked factor explaining the configuration of falcons' ocean-crossing corridors.