Nocturnal migratory flight initiation in reed warblers Acrocephalus scirpaceus: effect of wind on orientation and timing of migration

We used radio-telemetry to study autumn migratory flight initiation and orientation in relation to wind and air pressure in a nocturnal passerine migrant, the reed warbler Acrocephalus scirpaceus at Falsterbo, southwest Sweden. The majority of the reed warblers departed in the expected migratory direction towards south of southwest, while a low number of the birds took off in reverse directions between north and east. Flight directions at departure correlated with wind directions. These correlations were particularly prominent at higher wind speeds but were absent at wind speeds below 4 m/s. Birds departing in the expected migratory direction compensated completely for wind drift. The reed warblers preferred to depart during nights with tailwinds and when air pressure was increasing suggesting that reed warblers are sensitive to winds and air pressure and select favourable wind conditions for their migratory flights. Since air pressure as well as velocity and direction of the wind are correlated with the passage of cyclones, a combination of these weather variables is presumably important for the birds' decision to migrate and should therefore be considered in optimal migration models.