Curry-Lindahl, Kai (1982): Das große Buch vom Vogelzug (German; The Big Book of Bird Migration) Berlin-Hamburg, Verlag Paul Parey, 208 pp.
(APUSlist No. 3225)
by ULRICH TIGGES & EDWARD MAYER
This work gives us an overview of the complexity of bird migration. As well as covering topics such as “Which birds migrate” or “Orientation and navigation” it deals with global and regional problems, as well as with differences in behaviour, e.g. migration of single birds or the migration of flocks, and the altitude and speed of bird migrations. It also deals with some specific problems, especially concerning migration routes.
One can find the following details there for the Common Swift Apus apus.
As a long distance migrant, the Common Swift places little reliance on weather conditions. Its impulse to migrate relies on inner factors, including an “inner clock”. Sudden changes of weather may accelerate or decelerate the start of the migration, but this influences the migration only when the bird is ready to migrate.
Apart from the annual migration, the Common Swifts may perform so called “weather flights”. Since this species feeds exclusively on airborne insects, it relies on the presence of airborne plankton, which may disappear during bad weather conditions (e.g. long periods of rain and cold). The birds overcome such critical conditions by flying to more hospitable areas. They undertake such flights even when there are eggs or chicks in the nest. The young birds will then fall into a torpor, which saves their energy.
Common Swifts over-winter in tropical and southern Africa. The eastern subspecies (Apus a. pekinensis) uses the same routes to its breeding areas as they used originally after the ice retreated at the end of the last Ice Age.
A. a. pekinensis migrates preferably into the arid areas of Botswana, Southwest Africa and South Africa. Until now it has not been found west of Lake Chad. The western subspecies (A. a. apus) prefers the humid areas in East Africa. In general this subspecies is abundant in the Sub-Saharan Africa.
The Asian Common Swifts migrate in a western direction until they reach Chad, then turn southwards until they reach the Kunene River (on the border of Angola and Namibia). Here they turn south-east to the Kalahari Basin, which appears to be their main wintering area.
European Common Swifts depart their breeding areas in late summer, then go south-west until they reach the edge of the tree savannah in the south of the Western Sahara. From there they head south-east. Their wintering area lies from Nigeria-Liberia in the west to South Africa in the south and as far as Madagascar in the east. In the Senegal area the Common Swift does not over-winter, although it passes through the southern and northern areas there regularly.
In spring all Common Swifts migrate along the same routes with the exception of the Asian subspecies Pekinensis, which probably chooses the southerly route across Somalia.
An experiment in Sweden revealed that the Common Swift hunts for its food at altitudes of 1400 to 3600 m.