Paolo Becciu, Shay Rotics, Nir Horvitz, Michael Kaatz, Wolfgang Fiedler, Damaris Zurell, Andrea Flack, Florian Jeltsch, Martin Wikelski, Ran Nathan and Nir Sapir

White storks (Ciconia ciconia) are large birds that migrate from Central Europe to Sub-Saharan Africa. During their long migrations, they face several geographical barriers, such as ocean and seas. White storks, as well as other large birds, avoid passing over waterbodies because they use the energy of thermals forming on land to soar, and then move forward by gliding. Thermals are much weaker and scarcer over the sea; therefore, they may try avoiding sea crossing when possible. To investigate what causes sea crossing in white storks and the consequences of the birds’ choice, we studied storks that cross the area of the Iskenderun Bay in the northeastern corner of the Mediterranean Sea. We predicted that tailwinds, time of passage, age and sex could affect sea-crossing behavior. Furthermore, we expected that crossing the bay could help the storks to reach their migration goal faster by shortcutting the area of the Iskenderun Bay, at the cost of using a more energetically costly flapping flight over the sea, compared with soaring flight over land. We found that westward and southward winds promoted a cross-bay journey in spring and autumn, respectively, acting as tailwinds. Additionally, sea crossing was associated with flapping flight and higher flight-related energy expenditure, as well as higher ground speed than travel over land. Notably, daily travel distances of spring migrants that crossed the bay were 60 kilometers longer than those of land-detouring birds, allowing them to reach their destination faster but likely incurring a higher energetic flight cost of their travel. Our findings confirm that atmospheric conditions can strongly affect bird route selection. Consequently, migration timing, speed and movement-related energy expenditure differed considerably between the two migratory seasons and the two route choices, highlighting a time-energy trade-off in the migration of white storks.

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