Common Swifts are extreme long-distance migrants and winter in equatorial Africa as far as eastern South Africa. No precisely delimited wintering area is known. The swifts, which are presumed to fly continuously, appear to roam over a vast area depending on the weather situation (GLUTZ and BAUER, 1980). The stay in the breeding area and in the winter quarters normally lasts about three to three and a half months in each case. Breeding sites in Central Europe are usually left once the young have fledged, mostly in the second half of July or in early August. The final departure may extend until the end of August; even after that, individual passage migrants still occur (probably from more northerly breeding areas), and the latest observations in Central Europe date from around 20 November. The predominant direction of autumn migration is southwest to south.
In poor weather conditions, migrating swifts readily follow the courses of rivers, where they are more likely to find food than in the surrounding countryside. Higher mountain ranges are probably crossed in spring and late summer mainly in fine weather. The populations from Central and Western Europe migrate chiefly via Spain and North-West Africa, whereas swifts from South-Eastern Europe and Russia travel through the eastern Mediterranean. The position of the migratory divide, or of the zone where the two populations mix, is still unclear (GLUTZ and BAUER, 1980). On their way south, the birds follow the North-West African Atlantic coast in large numbers, but they also fly straight across the Sahara and then, from the moist savannahs, east-south-eastwards into their wintering grounds in Central Africa.
The return migration follows the same route, but observations of spring migration are relatively scarce both in Nigeria and along the Atlantic coast. Swifts migrate singly, in small parties or even in vast aggregations (GLUTZ and BAUER, 1980).
Cyclonic weather flights are regarded as a special form of swift migration. The birds avoid bad-weather fronts by making evasive flights that may cover thousands of kilometres and last from days to weeks. LACK (1956) describes how swifts, unless they retreat into their nest holes, escape approaching thunderstorms by flying along ahead of the front, later flying back into the wind through the belt of rain, so that they are exposed to the precipitation for as short a time as possible. In doing so they form flocks that can reach enormous numbers of several tens of thousands of birds. This flight from bad weather has long been known from all parts of Europe. Not infrequently breeding birds take part as well, since their young are able to survive an absence of their parents lasting several days by falling into torpor.