Individual And Sex-Related Patterns Of Prolonged Flights During Both Day And Night By Great Reed Warblers Crossing The Mediterranean Sea And Sahara Desert

A wide variety of the barrier crossing strategies exist among migrating songbirds, ranging from strict nocturnal flights to non-stop flights over a few days. We evaluate barrier crossing strategies in a nocturnally migrating songbird crossing the Mediterranean Sea and the Sahara Desert, the great reed warbler, exploring variation between the sexes and within individuals. We used data from 31 year-round light-level geolocators tracks from 26 individuals (13 males and 13 females), with four individuals tracked for 2-3.5 consecutive years. Almost all individuals (25 of 26) prolonged their flights into the day at least on one occasion. The mean duration of these prolonged flights was 19.9 h and did not differ between sexes or seasons. Fifteen birds performed non-stop flights during more than one full day and night (>= 24 h; mean = 31.9 h; max = 55 h) in autumn and/or spring, but these flights were generally too short to cross an entire barrier (such as the Sahara Desert) in one non-stop flight. Patterns of prolonged flights showed considerable within-individual variation in females between seasons (autumn versus spring) and in both males and females between years, suggesting high individual flexibility in migration strategy. Significantly more males than females performed prolonged flights during autumn migration, but not spring, possibly reflecting sex-specific carry-over effects. We conclude that great reed warblers have the ability to conduct prolonged continuous flights for up to several nights and days, which potentially would allow them to cross the Sahara Desert in one non-stop flight. However, they typically use a mixed strategy of several nocturnal flights with intermittent stopovers in combination with 1-3 prolonged flights. Prolonged flights covered less than half (44%) of the total flight time across the barriers, and the diurnal parts of the flights covered only 18% of this time.