Barrier effects of mountain ranges for broad-front bird migration

The infuence of mountain ranges on broad-front bird migration has been investigated for several decades. However, almost all the studies have focused on fight behaviour, and have rarely considered the quantitative efect 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 fndings clearly show that topography signifcantly infuences the spatial distribution of bird migration. Most birds few 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 difer considerably from the pattern in the foreland of the Alps. This may be infuenced by the wind conditions, causing birds to enter valleys of diferent geographic orientation. At a large scale, we did not fnd any evidence of a major barrier efect. In contrast to the south-west oriented central part of the Alpine Arc, where birds concentrate along the mountain ranges, the fux of autumnal bird migration appears to fow over and through the nearly perpendicularly oriented (with respect to their migratory direction) Eastern Alps. This demonstrates how the infuence of mountain ranges as barriers depends on their spatial context.
Keywords: Radar, Alpine arc, Migration intensity, Topography, Collision risk