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Flight Altitude of Migrating European Honey Buzzards Facing the Open Sea

Abstract Migrating birds adjust their behaviour in order to reach their final destination safely and in a timely manner. In doing so, they fly at different altitudes, but unlike passerines, raptors do not explore all air levels searching for the best tailwind assistance. Soaring species migrate over the mainland using updrafts to optimize soaring-gliding flight and reaching higher altitudes during midday. However, there is little information on which variables affect their flight altitude when facing the open sea, where thermals are very weak and they are forced to use powered flapping flight for a long time. To fill this gap, we recorded the flight altitude of migrating European Honey Buzzards Pernis apivorus as they crossed the Tyrrhenian Sea (Central Mediterranean) and passed over a small island. During four migratory seasons, we recorded the altitudes of birds when they reached the NE coast of Ustica, a volcanic island between Sicily and central Italy, and analysed data in relation to several variables. The results showed that the tendency to continue migrating, flock size, and wind speed are the most important features in explaining height variation in migrating European Honey Buzzards facing the open sea.