Topography and wind moulding directions of autumn migration between Europe and the West African savannas

Abstract

This review on autumn migratory directions is thought as a complement to an earlier overview on the vertical distribution of bird migration between the Baltic Sea and the Sahara (Bruderer et al. in J Ornithol 159:315–336, 2018): (1) A broad synopsis shows that nocturnal migration is generally SW-oriented above the western half of the European mainland, bending gradually southward above the western Sahara and shifting SE at the Sahara-Sahel transition. Important deviations of the SW stream occur along mountain ranges and coastlines. (2) Taking the Alps as a case example of such a leading-line reveals that the effect of the Alpine arc on migratory directions increases from E to W, becoming most prominent in Switzerland where the northern border and the main ridges of the Alps bend from WSW towards SSW. Thus, the migratory stream gets increasingly aligned with the course of the mountain range and reaches highest concentrations in the Swiss Lowlands. (3) Simultaneously recorded tracking radar data on nocturnal migration above Southern Germany and above the Swiss Lowlands show similar distributions of headings, but different tracks (flight directions over ground). (4) Generally, a large proportion of the tracks above the rather flat country N of the Rhine is shifted towards S or SE by frequent westerly winds. This contrasts with barely drifting birds facing south-westerly headwinds canalized along the Jura Mountains in the Swiss Lowlands. (5) Tracks and headings under varying wind conditions above Southern Germany visualise different reactions to following vs opposing winds as well as to side-winds from the right and left. (6) Radar-tracked night migrants above three different sites in south-western Switzerland show their reactions to different topographical conditions which vary from moderate leading effects of the Jura Mountains at a lowland site, to extreme funnelling at an Alpine pass, and wide scatter when a large Alpine valley perpendicular to the principal SW-direction of migration is crossed. (7) Distinguishing between three height zones reveals that (a) the proportion of SSW migration increases with height; this besides a few birds drifting across the Jura Mountains; (b) at the Alpine pass, forward migration is canalised as a narrow stream and complemented by notable reverse movements, while the highest level (above the crests) is characterised by wide directional scatter including moderate southward drift; (c) the proportion of movements along the SE–NW leading Rhone Valley decreases with altitude, while the proportion of SW migration increases, and the distribution approximates that at the pass in the highest zone. (8) This information leads to ideas for continuative studies, particularly on reverse movements, drift and compensation in the Alps and their northern approach areas.