How do diurnal long-distance migrants select flight altitude in relation to wind?

To save energy and time, migratory birds are expected to select time periods and flight altitudes with favorable wind conditions. In spring 2006, we studied diurnal migration using tracking radar at the Strait of Messina. A total of 1530 radar tracks were analyzed with respect to flight altitude and wind conditions. The tracks included Honey Buzzards, Marsh, Montagu's, and Pallid Harriers, Black Kites, falcons, swallows, swifts, and herons. Maximum flight altitude recorded was 2495 m above ground level (agl), but 90% of birds were flying below 1135 m. All species investigated showed clear evidence that tailwind assistance at the bird's flight altitude was better than below and similar or better than just above. However, the birds did not select the best flight altitude with respect to tailwind assistance within a given height range of 1000 or 2500 m agl. More likely, they were selective for the first optimum they encountered when climbing, even when better winds occurred at higher altitudes. Understanding the selection of flight altitudes by diurnal migrants is essential for modeling and predicting the temporal and spatial concentration of migratory birds. This kind of information is urgently needed for the evaluation of collision risks in relation to the impact of wind turbines on birds but also for assessing aircraft safety.

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