Strandskadetrækkets dagsrytmer ved Blåvandshuk og Revtangen i Norge

摘要

Siden Dansk Ornitologisk Forening oprettede Blåvand Fuglestation for over 50 år siden er der offentliggjort adskillige artikler om det meget store træk af vadefugle (Netterstrøm 1970, Thelle 1970, Meltofte et al. 1972, 2006, Noer 1979, Meltofte 1988, 1993, 2008), og ikke mindst er sammenhængene mellem trækket og vejret blevet undersøgt (Meltofte & Rabøl 1977, Meltofte et al. 2018). Diurnal rhytms of the migration of Eurasian Oystercatchers Haematopus ostralegus at Blavandshuk, Denmark, and Revtangen, Norway In 1972, continuous visual observations of wader migration from sunrise to sundown were carried out at Blåvandshuk, the westernmost point of Denmark, during the period 20 July to 25 August. In 1973 these observations were repeated from 28 July to 18 August in combination with observations at Jaerens Rev (Revtangen) in SW Norway. The purposes were 1) to establish daily temporal distributions of wader migration at Blåvandshuk, and 2) to investigate the relation between the migration in SW Norway and W Denmark. For various reasons the full results were not published until recently (Noer 2017), but even if old, these data are still unique. No corresponding observations have been carried out since then. The paper analyses the migration of Oystercatchers. In 1972, a total of 50160 Oystercatchers were counted at Blåvand. In 1973, 23887 and 18785 were counted at Blåvand and Revtan-gen, respectively. On nine of the 22 days, the observations at Revtangen covered the full day. On the remaining 13 days, observations invariably covered the first five hours after sunrise and the last three before sunset. Daily temporal distributions of migration differed widely between the two localities. At Revtangen, numbers passing increased after sunrise and peaked 1-5 hours later, thereafter decreasing (Figs 1,3 and 5). At Blåvand, migration intensity was generally high during the first five hours after sunrise, at a daily minimum during the following five hours, and then again increasing during the afternoon (Figs 3 and 4). At both localities, migrating flocks tended to pass in waves, and these waves were most pronounced on days with large numbers of migrants. The results from Revtangen clearly suggest that most of the observed migration was flocks that had taken off about sunrise from localities 50-300 km further to the north (Fig. 7). The most likely interpretation of the more complex results from Blåvand is that the migration during the morning hours is dominated by birds that have taken off at sunset the night before and migrated throughout the night. After sunrise the following morning, they continue their flight until they reach their destination in the Wadden Sea. The increase in intensity during the afternoon is thought to consist of diurnal migrants having taken off at sunrise the same morning. Undoubtedly, many of these birds come from Norway via Revtangen (see also Meltofte et al. 2018). Taken together, the results suggest that many, probably most, Oystercatchers breeding in northern Norway stage at localities 50-300 km north of Revtangen before carrying out the last leg of the migration to the Wadden Sea (Fig. 7), covering a distance of 600-900 km in one go. Part of the migration at Blåvand, however, occurs in easterly wind directions. These birds are thought to belong to the Baltic/ White Sea populations (see Meltofte et al. 2018), and the most likely recruitment area is the Southern parts of the east coast of Sweden (Fig. 7).