5. Uitgebreide soortsbespreking
Club van Zeetrekwaarnemers Verslagen , Volume 28 - Issue 1 p. 5- 12
As in the previous two half yearly reports, treating the movements of sea- and shorebirds along the Dutch coast, some species have been selected to be treated a little less superficially than is customary in these reports. Divers, Great Skua and Arctic Skua have been selected as a result of their unusual frequency of occurrence in the autumn of 1984 along the Dutch coast, while the Lesser Black-backed Gull is discussed for no special reason at all. ‘Divers’ (including Red-throated, Black-throated and unidentified birds, belonging to either of these two species) were unusually common during autumn 1984, with most observations originating from the Wadden Sea area(Wd) and Zuid Holland & Zeeland(ZH), while the numbers seen in Noord Holland(NH) did not differ remarkably from previous autumns (see table 1).Divers were observed regularly from October onwards, especially during November and December. The largest numbers were observed during the first two days of December, during the second decade of December and during the last few days of this month. Especially on December 1 unprecedented numbers were recorded on Texel(Wd): 1105 flying southwards in 3 hours. Most birds were observed flying to the south(west); only in NH there was appreciable fraction of the birds observed to be flying northwards. This phenomenon has attracted attention before in previous years and for a large number of species. Most periods of ‘diver’- movements coincided with calm weather, high barometric pressure and (south-) easterly winds. For a survey of the autumn-migration, see fig.5.1.1. Blackthroated Divers were, as usually, scarce and most, specifically identified, ‘divers’ proved to be Red-throated Divers, which showed a similar occurrence pattern to the one of ‘diver’. When looking at table 1 and noticing that autumn 1984 brought many ‘divers’ within sight of the coast, one tries to find out whether there were indeed more ‘divers’ than usual along the Dutch coast, or whether the observed phenomenon is caused by some kind of counting artefact, i.e. whether during autumn 1984 the amount of observation-hours, made in November and December, the months in which most ‘diver’ -movements occur, has been disproportionately large in comparison with the preceding autumns. Table 2 shows that the fraction of observation-hours, made in November and December, has remained remarkably stable during the years, so it seems indeed likely that the autumn of 1984 brought unusually large numbers of ‘divers’ along the Dutch coast. Table 1 seems to suggest that ‘divers’ have been increasing in numbers, especially in Wd, ever since autumn 1979. However, there are no known changes reported recently in the wintering areas, NE of the Netherlands, whether in the size of the wintering population, whether in the coastal waters, used for wintering. As a result of the Delta-works, however, there have been some recent changes in the coastal waters off Zeeland (southwestern Netherlands), that may have been of influence on the number of wintering ‘divers’. Autumn 1984, as well as autumn 1983, showed unusual numbers of Great Skuas along the Dutch coast. In contrast to the preceding autumns, during the last two autumns most Great Skuas were seen in NH, while the birds are usually mostly seen in ZH. Table 3 shows the number of Great Skuas, seen along the Dutch coast, from 1974 onwards. Although there were several July and August-observations, the first wave of Great Skuas reached our coast in the second week of September, mostly during stormy weather with westerly winds. In October the Great Skua was regularly recorded, but only occasionally were there over ten birds a day observed. Even during November and December, the species was fairly regularly observed, with the last record on December 21st. The Great Skua is presently an expanding species, both numerically and spatially (Harrison, 1983; Meek et al.,1985). This expansion cannot properly be observed in the occurrence of the Great Skua along the Dutch coast (see table 3). In southern Scandinavia however, there has been a recent increase in numbers of birds observed. which is explained by reference to the population expansion of the Great Skua (Rasmussen, 1981). Usually most Great Skuas are seen in September and October, though sometimes fair numbers may be seen in November; the latter was not the case in 1984. Most birds, seen in September and October, presumably consist of a mixture of non-breeders and Scottish birds; however, it is suggested, along with Rasmussen (1981 ), that the non-yearly observed flux of Great Skuas in November, may well be of Icelandic origin. Observations of Great Skuas after September, which are quite common, somewhat contradict Tasker et al. (1985), while observations of birds in November and December seem rather exceptional compared with Rasmussen (1981) and Tasker et al. (1985), although such records are not really unusual, according to Dutch standards. Autumn 1984 was an extremely good autumn for the Arctic Skua; only in autumn 1978 was there a slightly higher total, but the hourly mean was much lower. In July the Arctic Skua was observed only twice, while the species was already quite regularly seen in august. In the second week of September large numbers passed the Dutch coast, coinciding with depressions and heavy westerly winds and rain. The last week of September also brought fair numbers within sight. In October two days (7th & 20th), with westerly storms, rendered large of Arctic Skuas within coastal view. In November the species was still regularly observed, especially in ZH. The last record was on December 3rd. Although most birds were seen in NH, as a result of the enormous peak-numbers, the species was much more regularly observed in ZH. As the Arctic Skua is not, solely, dependent on cyclic or irregular food-sources, as voles or lemmings, for its breeding success, there is no simple explanation for an irruptive occurrence of the species along our coast. The percentage of juvenile birds in autumn does not seem to fluctuate strongly among the years (ca. 20%), Meltofte (1979) states that, by comparing the light- dark-phase-ratio, it is possible to find out the breeding origins of the birds. For ‘our’ birds, this proved not feasible, as a result of insufficient accuracy in the plumage determinations. Nevertheless, it is carefully suggested, that birds, seen in august and early September, are of Scottish origin, while the birds, seen from half September onwards, may very well be of Scandinavian or even arctic origins. As a result of different observer-criteria and the differing ease of identification, it seems hardly sensible to try to interpret the observed plumages and its implications for the origins of the skuas and the differences between areas. In autumn 1984 no unusual numbers of Lesser Blackbacked Gulls were seen, but the species was remarkably scarce in NH, and rather plentifully seen in ZH (see table 5). Until October the gull was common in NH and ZH, but in Wd the numbers already started to dwindle in September. In November the gull was still fairly regularly seen in NH and ZH, but in December the species was only observed with some regularity in ZH. This matches the results of Blake et al. (1984) and Platteeuw (1985). Of the three subspecies of the Lesser Black-backed Gull, which occur in Europe, only L.f. graellsii and L.f. intermedius are regularly seen along the Dutch coast. Of these two, only L.f. intermedius is seen in any numbers. Both subspecies winter from Iberia to West Africa, but in recent years there has been a development towards more northerly wintering and earlier return towards the colonies, at least in England (Cramp & Simmons, 1983). In the Netherlands however, there are as yet no indications pointing in this direction. The autumn migration in 1984 showed its first wave in late July/early August in ZH. By the end of august and the beginning of September migration really started to take shape, all along the coast. During the remainder of September and the middle of October substantial southward migration occurred in ZH, and, in September only, in NH. Juvenile birds (13%) were observed mainly in September and October, while immatures (3%) and adults (84%) were more evenly distributed throughout the autumn.
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