Barrier effects of mountain ranges for broad-front bird migration

The influence of mountain ranges on broad-front bird migration has been investigated for several decades. However, almost all the studies have focused on flight behaviour, and have rarely considered the quantitative effect of mountain ranges as a barrier. The present study investigates the spatial and temporal pattern of autumnal bird migration across the eastern part of the Alpine Arc (Austria). Migration intensity was recorded continuously using two dedicated bird radar systems during a full autumn migration season. One radar was located to the north in the Alpine Foreland region for the whole observation period, serving as a reference for the number of birds approaching the Alps. The second radar was moved between four locations across the Alps, operating for 21-26 days at each site. Our findings clearly show that topography significantly influences the spatial distribution of bird migration. Most birds flew lower than the main ridges, with only a small proportion detected above such altitudes (~ 3000 m above sea level). The daily pattern of migration was similar across sites, and there was no accumulation of birds ("Zugstau") in front of the Eastern Alps. At only one of the inner-Alpine sites did the day-to-day pattern of migration intensities differ considerably from the pattern in the foreland of the Alps. This may be influenced by the wind conditions, causing birds to enter valleys of different geographic orientation. At a large scale, we did not find any evidence of a major barrier effect. In contrast to the south-west oriented central part of the Alpine Arc, where birds concentrate along the mountain ranges, the flux of autumnal bird migration appears to flow over and through the nearly perpendicularly oriented (with respect to their migratory direction) Eastern Alps. This demonstrates how the influence of mountain ranges as barriers depends on their spatial context.