5. Uitgebreide soort bespreking
Club van Zeetrekwaarnemers Verslagen , Volume 26 - Issue 1 p. 5- 13
In this first part of the report of autumn 1983 some species are treated in more detail, because of remarkable aspects of their occurrence in this period deserving some more attention than the usual superficial treatment. This season the petrels were unusually numerous, while the Gannet, which normally follows a rather simmilar trend, appeared in remarkably low numbers. For these reasons the petrels as well as the Gannet have been selected for special treatment. Furthermore because of some mass migration of Eiders observed in the second half of October on the Wadden Sea island of Schiermonnikoog, this species as well was selected for treatment in this first part, although total numbers per hour were not exceptionally high. Far the most numerous of the petrel-family this autumn along the Dutch North Sea coast was the Fulmar. This was particularly due to an extra ordinarily large-scale movement on the 12th and the 13th of September, when for example at the Hondsbossche Zeewering in Noord-Holland 6662 individuals were observed in 12 hours flying mainly northwards. Other sides also reported exceptionally high numbers those days, although in the morning of the 12th in Zuid Holland most birds flew south. This was probably caused by the SW wind in the morning which later on changed to NW, so the Fulmars, which usually prefer headwinds (Camphuijsen & Van Dijk 1983), changed their flying direction accordingly. The figures 5.2.1. and 5.2.2. show for the three areas Zuid Holland, Noord Holland and Wadden Sea area respectively how exceptional autumn 1983 was. The mean numbers per hour had never been so high in either of the three areas, but the percentage of hours in which a Fulmar was recorded had been higher in Noord Holland in two previous autumns. This means that in this area, although total numbers reached unprecedented levels, the frequency of occurence was not exceptionally high. The high numbers were almost entirely caused by the mass movement on the 12th of September. In the two other areas Fulmars have been more frequent and numerous than usual during the entire month of September. October and november produced much lower numbers and the last birds were observed on 30th of december on Texel (cf. figure 5.2.3.). Considering the estimated 440,000 breeding pairs of Fulmars in North Sea colonies and the fact that the species is still getting more numerous every year (Blake et.al. 1984) we might expect large-scale movements such as described above (caused by some special hard weather conditions) to occur more frequently in the future. September/october will be the best month, because then the first year birds have left the colonies and have not yet moved out of the North Sea, in which they return in their 3rd or 4th year (Blake et.al. 1984). Manx Shearwaters occured in fair numbers (346 individuals), with the majority being seen from the sites in Zuid Holland (72.8%). The majority moved southwards, the best day being the sth of September in West-Kapelle. No less than 71 birds were observed flying south and one north during 11 hours of observation. Until half October occurence was frequent. The last Manx Shearwater was seen on the 20th of October. For the first, time since 1979 there was a rise in the number of Sooty Shearwaters recorded. Figure 5.4.1. shows the seasonal pattern for each of the three regions. In all areas fair numbers were seen, although the Wadden Sea area seemed a little bit less fafoured than the other two. The majority flew by in the first half of September and the second week of October. November only showed one Sooty Shearwater, but a northwestern gale on the 26th of december brought the last bird of this autumn within sight. As usual only very few Storm Petrels were observed. Much more numerous was the Leach’s Petrel of which 239 individuals have been recorded. Never in Dutch seawatching history had a number like this been reached. Figure 5.6.1. shows the seasonal pattern of occurrence. The first bird was seen on 3th of September and from until the 22nd, the best day being the 12th when in Scheveningen (Zuid Holland) 3 individuals flew south and 2 north. October as well produced some good days like for example the 11th with 16 south at Scheveningen and the 28th with 16 W at Schiermonnikoog. In november 6 birds were recorded and quite exceptional was the occurrence of 3 Leach’s Petrels in the heavy westerly storm period in the last week of december. In spite of all the interesting westerly storms which brought us so many petrels the Gannet had not been seen in such low numbers since autumn 1975. Especially in the southernmost region of Zuid Holland very few Gannets were recorded (786 birds, 0.86 individuals per hour). Furthermore there were hardly any records of foraging, non-migrating birds. For example in the Wadden Sea area only 1.8% of the grand total was recorded as nonmigrating, while in normal autumns some 10% of all birds do not migrate. Figure 5.7.1. shows that the bulk of passage took place in September in Noord Holland and in the first week of October in the Wadden Sea area. In all three areas some 90% of all Gannets were identified per age-group. In Zuid Holland more first-year birds and less adult birds were seen than in the two northern regions. This fact, together with the remarkably low numbers in this area, might indicate that the coastal waters of the southern part of Holland is a marginal area which the fittest Gannets avoid. Out at sea at the same latitude twice as many Gannets were recorded during counts from a platform (Camphuijsen et.al. 1982), so the paucity of Gannets would be limited to the coastal zone. Figure 5.7.2. shows the age composition during the autumn of 1983. This is the well-known picture of a high percentage of immature birds in summer and early autumn decreasing rapidly in October, when the adult birds leave the colonies and many of the juviniles and immatures move south (cf. Landsborough Thomson 1974, Nelson 1978). A rise in first year birds early September probably reflects exodus of juviniles from the colonies, while first-year birds from august should be 2nd calender year birds retaining their brown heads. Observation In Blavandshuk shows much lower numbers of juvinile and immature birds especially in august (Meltofte & Overlund 1974). Apperantly the Dutch coast lies still within reach of the summer fouraging range of non-breeding birds while the Danish is to far east to be reached frequently by these birds. Eiders did not occure in exceptionally high nembers this autumn (figure 5.8.1.) Evidently most birds were seen in the Wadden Sea area (figures 5.8.1. and 5.8.2.), where especially in the last two week of October some impressive westward migration was observed. Swennen (1976) has demonstrated the importance of the Dutch Wadden Sea and especially the western part of it as a wintering area for Eiders from the Baltic region. It is therefore hardly surprising that, while thousands of Eiders can be observed flying westwards at the eastern island of Schiermonnikoog, at the islands and at the sites further south and west in Noord Holland and Zuid Holland only insignificant numbers fly by. Another remarkable difference is the main flying direction. While on Schiermonnikoog practically all Eiders move west, on Texel and in Noord Holland a high percentage is flying NE. This may very well be a course correction in order to reach the proper winter area after having been driven too far south and west. In Zuid Holland almost half of the Eiders observed were swimming along the coast and among the flying birds the majority flew SW. Apparently having reached that far south the Eiders do not attempt to return to the Wadden Sea anymore. Swennen et.al.(1979) regristered a very low percentage af first-year birds among those wintering in the Wadden Sea (± 3%). Furthermore among the adults they found 55% were males. This fits rather well with the 60% males regristered during sea-watches in the Wadden Sea area this autumn (n=13,928). Along the coast of Noord Holland and Zuid Holland however only 28% (n= 3,873) and 27% (n=551) respectively were recorded as males. Considering that females in general show a higher juvinile mortality that males (Swennen et.al. 1979) and that first-year males cannot properly be distinguished in the field it seems very likely that a large proportion of the Eiders observed south of the Wadden Sea consist of juvinile and immature birds.
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