Effects of Alpine Topography and Winds on Migrating Birds

Abstract

Radar tracks of about 150 single migrants and 2 to 3 measurements of upper winds per each of 139 nights in the lowlands of Switzerland and 35 nights at an Alpine pass show that the principal direction of migration is parallel to the mean course of the border and to the main ranges of the Alps and that winds against or from the right side of the main vector of migration induce pseudodrift (increased migration of northern and northwestern populations) while winds from the left are usually compensated (this to a lesser extent in poor visibility). The principal direction remains astonishingly constant at every altitude and seems not to depend on actually visible leading-lines, while in single flight paths close adaptations to topographical features are recog-nicable, most pronounced at lower levels and in unfavorable winds. A. Methods and Data The basic methods used for the present study were described by Bruderer (1971) and by Bruderer and Steidinger (1972). Some improvements introduced in the meantime affect only the way of recording (simulta-neously on magnetic tape and on a two-channeled XY-plotter) but not the sort and amount of data: About 150 night migrants and two to three pilot balloons have been tracked with the fire-control radar "Super-fledermaus" (Radar equipment made available by Contraves AG, Zurich, and by the Swiss Army) to obtain information on the distribution of bird tracks and wind directions in every height-band desired during each night of observation. The data used for this paper originate from 64 spring nights (1969 and 1971) and 75 autumn nights (1968 and 1971) in the lowlands of Switzerland. The radar sites used for these lowland studies were located about 15 km to the N and to the SW of Zurich (cf. Fig. 1), about 400 m ASL. Additional data were collected at the pass of Hahnenmoos, in the Bernese Alps (about 1950 m ASL); we included 30 nights of autumn 1974 and selected nights of autumn 1975. It is important to bear in mind that each of the diagrams on the spread of directions given in this paper represents the relative quantity of birds migrating toward each direction under the given conditions, but they do not include absolute numbers indicating the intenSity of migration during different nights.