Dusk and dawn ascend of the swift, Apus apus L.

Radar observations on bird movements over The Netherlands in June andJuly revealed spectacular nocturnal Swift concentrations over the IJsselmeerand to a lesser extent above other waterbodies in and around TheNetherlands at some kilometres from coastlines. The Swifts performcharacteristic dusk and dawn ascends up to altitudes over 1800 m, while theymostly disappear from the radar screen during midnight, apparently flyingbelow 300 m. These observations contradict the general story of Swiftssleeping on the wing high over their breeding areas. It is hypothesized thatmany Swifts perform feeding flights at night over open water to feed oninsects with aquatic larvae. Besides that, there, they socially join typicaltwilight ascends for orientation purposes.

Luit S. Buurma, INTERNATIONAL BIRD STRIKE COMMITTEE