Institut für Vogelforschung "Vogelwarte Helgoland", Inselstation
Hüppop K, Dierschke J, Dierschke V, Hill R, Jachmann KF&Hüppop O 2010: Phenologogy of the "visible bird migration" across the German Bight. Vogelwarte 48: 181-267
Within a project supported by the Federal Ministry for the Environment, Nature Conservation and Nuclear Safety dealing with possible consequences of offshore wind farms on bird migration observations of migrating birds were carried out on three islands in the German Bight, Sylt, Helgoland and Wangerooge, from autumn 2003 to the end of 2006. With "seawatching" the migration of mostly larger species (waterbirds in the broadest sense) over the sea was recorded
and "islandwatching" concentrated on the migration of smaller species (mostly passerines) over land (islands in this case). The particular goal of this study is the synoptic observation at three locations at least during the migration periods. Apart from some exceptions, at all three sites seawatching showed the highest migration intensity in the morning hours, much less migration occurred at midday and in the evening. Varying proportions of species are mainly caused by the different
breeding and staging/wintering areas, whereby the three sites are touched by the birds to a greater or lesser extent. This phenomenon is most obvious in geese.
With seawatching in the first three hours after sunrise a total of 185 species was recorded (154 near Sylt, 137 near Helgoland and 148 near Wangerooge). For 23 species, which were abundant or are regarded to be critical concerning effects of offshore wind turbines, the seasonal and daily patterns of migration intensity are described and shown in graphs, diurnal variation of migration intensity for 15 species and flock size for 11 species. In many species, migration was more
intense near Sylt in autumn and near Wangerooge in spring, whereas a more balanced relation of the two migration periods was typical for Helgoland. During both seasons the daily migration intensity of the three sites was highly significantly correlated. That holds true for the overall picture of all species as well as for the migration intensities of many particular species. Due to the geographical locations of the breeding and wintering areas of most species, a general direction of migration along a SW-NE-axis can be expected. However, the results of seawatching could prove this only for Helgoland. Near Sylt and Wangerooge, most birds followed the coastline, i.e. they migrated along a S-N-axis and W-E-axis, respectively.
With islandwatching in the first three hours after sunrise a total of 189 species was observed (165 above Sylt, 133 above Helgoland and 161 above Wangerooge). Phenology are described and presented graphically for 23 relatively abundant species, flock size for 10 species. As with seawatching, migration intensity was lower at Helgoland than at the two coastal islands and showed roughly the same amount of birds during spring and autumn migration. As a consequence of the preference of most
passerines to migrate along a leading line, migration was stronger for many species in autumn at Sylt and more pronounced in spring at Wangerooge: While relatively few birds headed towards the sea at Sylt (autumn) and Wangerooge (spring), the great majority of birds followed the coastline southward (Sylt in autumn) and eastward (Wangerooge in spring), respectively. Obviously, passerines avoided to fly towards the open sea during daytime and followed the coastline instead. Hence, the few birds arriving from sea
produced low migration intensities at Sylt in spring and at Wangerooge in autumn. The SW-NE-direction predominating in bird migration in Central Europe was only approved at Helgoland. But even there, jetties of the harbour acted as leading lines, because in autumn many passerines left the island towards SE and S. Because of the leading line effects at Wangerooge (spring) and Sylt (autumn) the
connection was strongest between the sites representing migration over sea, i.e. between Helgoland and arrivals above Sylt in spring, but between Helgoland and arrivals above Wangerooge in autumn. This held true for all species together as well as for the single species.