OFFRINGA H (1996) Bad weather-movements of Swift Apus apus at sea, 1980-94. LIMOSA 69 (1): 1-8.

Common Swift Apus apus Adult breeding and young nonbreeding common Swifts Apus apus have different strategies for coping with periods of unfavourable weather during the breeding season. Breeders stay on the nest during daytime, but young nonbreeders may perform extensive midsummer bad weather-migrations, flying south to southwest in an area hundreds of kilometers southeast of the core of an eastward moving frontal depression. Moving clockwise, the birds probably return north behind the depression. At-sea observations of Swifts were extracted from ESAS (European Seabirds At Sea Database) and from records collected during the SCANs-project (Small Cetacean Abundance in the North Sea, July 1994). In order to exclude spring and autumn migrants, observations made between June 10 and July 15 were selected. This produced 163 Swifts observed at sea during 102661 km travelled, 1980-1994: 98 in the North Sea, 47 during one survey in the Channel, Celtic Sea and Bay of Biscay (1989), and the remainder mainly in the Channel. Most Swifts flew solitarily; average flock size was 1.3 ? 1.49. Most observations were made within 300 km from the coastline and during morning hours. Combining observations clustered in time and space, 16 groups were distinguished (Tab. 1). Comparison with daily weather reports showed that each group of observations was related to approach of a frontal depression. Most of these depressions (N=15) came from the Atlantic Ocean and moved east reaching Ireland and Great Britain first. Swifts observed west of the British Isles and in the central North Sea probably originated from these countries. The predominant flight-direction was south (29 of 68 records); Swifts observed in the Bay of Biscay did not show a preferred direction of flight. Two groups of observations in the Bay of Biscay suggest birds were on a return flight, flying north behind frontal zones. About 97% of all Swifts recorded were located east or southeast of the centre; of an approaching depression (average distance to centre: 1300 ? 600 km). Swifts may fly very close to a frontal zone with precipitation; the average minimum distance is 380 ? 320 km east or southeast of the front, but two groups were than 70 km away. It is suggested that Swifts may also fly off to sea in order to avoid relatively brief periods of rain associated with rapid passage of a front, for it rains less at sea than ashore.

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