5. Uitgebreide soortbesprekingen

In this report on the passage of seabirds and coastal birds along the Dutch North Sea coast in the first half of 1985 four species have been selected for more extensive treatment, e.g. Great Crested Grebe Podiceps cristatus, Manx Shearwater Puffinus puffinus, Oystercatcher Haematopus ostralegus and Bar-tailed Codwit Limosa lapponica. The numbers of Great Crested Grebes seen in the first half of 1985 were exceptionally high. This was caused exclusively by the severe winter with persistent frost periods in January as well as February. In order to provide a very concrete idea of what this frost meant to the grebes, figure 5.1.2 presents the daily fluctuations of the percentage of ice coverage on the IJsselmeer, the largest body of fresh water in the Netherlands. As had been shown in other severe winters (for example 1978/79 and 1981/82) the ice coverage of most of the inland waters , and especially that of the IJsselmeer, leads to massive southward cold-rushes of Great Crested Grebes along the North Sea coast. Figure 5.1.1 with the hourly averages per 7-day period shows that this winter was no exception and that the most pronounced migration took place in the first two weeks of January, just before the freezing over of the IJsselmeer on 10 January. Especially on 8 January unprecedented numbers (12 698 ind. in only 3 hours) flew south at Scheveningen (Zuid-Holland). As usual in cold-rushes most Great Crested Grebes were recorded off Zuid-Holland, though the differences this year were probably a little exaggerated because of the lack of observations from Noord-Holland in the second week of January. Off the WaddenIsles numbers were invariably insignificant (see figure 5.1.1). Since the ice coverage of the IJsselmeer persisted until the beginning of March, the entire months of January and February fairly large numbers of Great Crested Grebes stayed offshore in the coastal waters. Nevertheless, the majority of the birds seemed to have left the country in the beginning of January. As usual too in severe frost periods of any length, after a two week or zo stay at sea large numbers washed up dead on the beaches. This winter almost 1000 corpses were found, of which a mere 44% (n= 856) was externally oiled. Almost invariably these birds had died of exhaustion. Apparently the Dutch offshore waters are no suitable alternative for wintering Great Crested Grebes in case of freezing over of their normal winter quarters. Thus the intriguing question arises whether the birds that do stay on (not moving further south) are either unexperienced (e.g. young) or so-called “low-quality” birds. More detailed investigations of the washed-up corpses in the future might shed some more light on question. Strangely enough the real spring movement, the return to the breeding territories, is never witnessed along the coast, neither in “normal” years, nor after severe winters. Possibly they fly north at night or over land. Thus 1985 as well did not show any significant numbers of Great Crested Grebes after the end of February. Although during the first half of 1985 not as many Manx Shearwaters occurred as in the same period in 1984, the fact that those who did turn up did so in much the same way as in other springs. Hard onshore winds in June have resulted more than once in fair to large numbers of Manx Shearwaters for Dutch standards, so this phenomenon is receiving some attention here. Table 5.2.1 summarizes the number of sightings in each of the six first months of the year throughout the period 1974-1985. It is evident that June is the only month in the first half of the year in which any numbers of this species are reported along the Dutch coast. Virtually all June records have been reported during northwesterly storm depressions and, as a matter of fact, during all such depressions which occurred in June throughout the years at least some Manx Shearwaters were seen. May and July hardly ever show similar weather conditions. Answers to the question about where the birds come from and what birds they actually are, are only tentatively suggested. Presumably quite a lot of birds enter in the northwestern part of the North Sea in June. This is suggested as well by records in this month off the British east coast. It is very likely that the birds concerned are immature birds, since these arrive at the colonies from the end of May onwards and they are more inclined to move far from the colonies in search of food than breeding adults. In case of NW-circulations these North Sea birds would find themselves blown into more easterly regions like the Dutch coastal waters. The very occasional records earlier in spring probably refer to returning adults which lost themselves during their spring migration. Particularly in Zuid-Holland much more Oystercatchers were observed in the first half of 1985 than in any other year. Table 5.3.1, in which the hourly averages are presented from 1977 onwards, demonstrates this clearly. The severe cold period in January was to be held responsible for these remarkable numbers. Along the coast of Scheveningen huge numbers flew south, especially on 16, 17 and 18 January. During the real spring migration two distinct peaks were noticeable (see figure 5.3.1, in which the hourly means per 7-day period are represented). The first one in March probably concerns adult Oystercatchers returning to their breeding areas, the second peak may possibly refer to immature birds which mount in large numbers in the Wadden Sea. Virtually all days with reasonable Oystercatcher-migration coincided with weak or moderate winds between N and E. Visibility was generally low. Bar-tailed Godwits passed by in huge numbers along Zuid-Holland and Noord-Holland. Off the Wadden-Isles they were hardly seen, which indicates that these birds were moving into the Wadden Sea itself. Off Zuid-Holland the numbers passing by reached a new record, off Noord-Holland only spring 1980 showed a higher hourly average (see figure 5.4.1, which presents the average numbers per hour migrating north in the different years from 1977- 1985). Figure 5.4.2 shows that most passage occurred in the last week of April and the first week of May. This pattern has been stable throughout the years. It is remarkable that off Noord-Holland migration may start a little earlier. This probably reflects the earlier migration of the birds from the British estuaries across the North Sea towards the Wadden Sea. These Bar-tailed Godwits leave their wintering areas in March and April, while the Banc d’arguin in Mauritania (a major wintering area) is left at the end of April. These latter birds probably fly directly to the Wadden Sea (as is suggested by color-marking in spring 1985) and therefore have to take advantage of favorable winds in higher regions, since they do not accumulate enough fat reserves in Africa to fly constantly for more than 2 or 3 days. This may explain why most, migration in Holland is seen during NE-winds (head winds) in the higher regions. Only then the birds have to come down, within visibility range of sea watchers, in order to minimize the negative effects of the head winds. Figure 5.4.3 shows the relationships between the percentage of observation hours from the period of maximum Bar-tailed Godwit numbers and the hourly average reached in the same spring. In Noord-Holland there was a significant positive correlation, whilst in Zuid-Holland there was none. This suggests that in Noord-Holland the visible migration of this species is more predictable (e.g. less dependent upon weather factors, etc.) than in Zuid-Holland. This may be interpreted as additional evidence for the hypothesis that a large proportion of the birds off Noord-Holland has been wintering in Britain. This rather stable winter population does not depend so strongly on weather circumstances, since the distance they have to cover is clearly less than the distance to be covered by birds of African origins. These birds, on the other hand, will certainly fly high and well out of sight whenever SW-winds are prevailing at our latitudes in the last week of April and the first week of May.