Grote concentraties Roodkeelduikers Gavia stellata tussen Cuxhaven en Helgoland
Sula , Volume 9 - Issue 2 p. 75- 78
Ship-based surveys in the coastal waters of Denmark, Germany, The Netherlands and Belgium, have shown that these nearshore waters are the prime habitat for divers in the North Sea. An estimated total of 43 000 Red- and Blackthroated Divers Gavia stellata and G. arctica winter in the area, which is under the influence of riverine discharge. The habitat differs from more offshore waters by its comparatively low salinity and waterdepth, and by high turbidity and winter temperatures. The most important parts are the waters north of. and influenced by, the river Elbe (Skov et al. 1995). Very large groups of Red-throated Divers have been seen on several occasions between mainland Germany (Elbe mouth) and the island Helgoland in the German Bight. The locations where these groups were seen suggest that the divers are associated with temperature /salinity fronts that frequently occur in the transition zone between riverine and North Sea water masses. In the 1980’s, birders from Helgoland regularly noted concentrations of divers, up to a maximum of 1500, on ferry crossings between Cuxhaven (mainland Germany) and the island. Divers could be seen on every winter crossing, and concentrations of several hundreds were not uncommon. Such concentrations were not seen during dedicated diver surveys in the German Bight in the early 1990's, but in January 1994 the concentration was ’rediscovered’, again from the ferry Cuxhaven-Helgoland (figure 1). From the fact, that a group of similar size (700-950 birds) was seen on the same location twice in 6 days, and because earlier observations also concerned this location midway between Germany mainland and Helgoland, it is tentatively suggested that fronts in this area attract such numbers of birds regularly.
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