Delayed autumn migration of the Swift Apus apus from Finland in 1986

Abstract

In the autumn of 1986 a couple of thousands of northern European Swifts did not start on migration but stayed in southern Finland and on the eastern coast of Central Sweden. Most Swifts had left Finland by the end of August, but a small number returned in mid-September, and the latest were seen in the beginning of November. In the course of the autumn no new migration could be observed. The birds were all adults . They sank into a state reminiscent of wintering, and some individuals started moulting in October. The health of the birds deteriorated due to shortage of food, and the surviving birds perished in the beginning of November when the frost started. No decreases were observed in two closely followed breeding colonies in southern Finland in 1987, so it is suggested that the late Swifts were from central or northern Finland. The strict time limits that the northern populations of the Swift have for breeding do not allow delayed migration caused by exceptional weather conditions as happened in the autumn of 1986. The slow moult normally commences in Africa after the autumn migration. In the autumn of 1986 the delayed departure of some individuals was due to unsuccessful accumulation of fat deposits because of adverse weather conditions . After this the birds started to winter, and, the moult commencing, it was impossible to migrate.