5. Uitgebreide soort bespreking
Club van Zeetrekwaarnemers Verslagen , Volume 27 - Issue 1 p. 5- 11
In this seawatching report of spring 1984 the following species of birds have received special attention, because of remarkable aspects in their migration patterns or numbers: Teal, Shoveler, Knot, Dunlin and Little Tern. Dabbling ducks usually participate substantially in the spring movements recorded during seawatches in March and April, especially along the coast of Noord-Holland. This year showed particularly high numbers of Teal and Shoveler, though the timing was quite normal. Of both species the migration peak occurred in the first half of April, while the first individuals started to pass by in the second week and in the first week of March respectively. Table 5.1 shows that for both species 1984 produced more birds per hour than ever since 1980. It demonstrates as well that the occurrence of higher numbers in Noord-Holland than in any other area is consistent throughout the years. This fact suggests strongly that the majority of these migrants would originate from the British Isles. The already traditional May migration was rather more prominent this spring than in most others. As known from previous investigations the European Knots can be divided in two distinct populations (with probable subspecific status, pers. comm. C.S. Roselaar), one from Canada, Spitzbergen and NE-Greenland and another from Siberia. Since the first population leaves its West-European wintering areas mainly in March, the birds passing by the Dutch North Sea coast in May are almost certainly all of Siberian origin. These birds come from NW-Africa and pass rapidly by, practically without significant stops in the Netherlands (only small peaks in May are registered in either Delta area or Dutch Wadden Sea) to Schleswig-Holstein, where they stay on until their final departure to Siberia. All migration observed was concentrated in Zuid-Holland and Noord-Holland and amounted to maxima of c. 2000 birds on 11 May and about 2500 on 13 May, which is earlier than in most years. Dunlins too reached exceptionally high numbers this year. On average 23.7 birds were observed flying north per hour throughout the entire first half of the year. Most movements were concentrated in March, but as usual too in most springs some movement occurred as well in the first half of April and in May. As the passing by of the vast majority of Dunlins in March coincides with the departure of the wintering birds from the British estuaries, it is assumed that virtually all these birds belong to the alpina population migrating to north Fenno-Scandia and NW-Russia. Therefore the March peaks may be higher in Noord-Holland than in Zuid-Holland, caused by concentration effects of westward flying birds from Britain to the Wadden Sea along the Dutch west coast. Schinzii from the Baltic could participate as well (they start breeding rather early), but they should already be in summer plumage at this time of the year (pers. comm. C.S. Roselaar) and the March migrants certainly are not. Probably these schinzii birds pass by in the first half of April. May migrants could be schinzii from Iceland and SE.-Greenland and the latest ones even may belong to arctica from NE-Greenland, both following a more easterly course than most individuals of these populations. They may stay for some time in the Dutch Della area, but are unlikely to visit the Wadden Sea with any regularity. This could explain as well why in May, unlike March, in Noord-Holland less migration of Dunlin is observed than in Zuid-Holland. These bird populations come from the south (NW-Africa) and are heading NW. Although this spring almost twice as many Little Terns migrated north along the Dutch coast (except the Wadden island coast) than in any other year, it is argued that this was largely due to a larger proportion of observation hours in the peak period than in previous years. Figure 5.4.1 shows clearly that in the limited migration period of the bulk of Little Terns 1984 was as a result of frequent favourable wind directions (N-K) a reasonably good year, but certainly not exceptional. For the generally higher numbers in Noord- Holland than in Zuid-Holland is suggested as the most probable explanation that birds flying north continue doing so by night. So those groups passing by Dungenes one day, fly along Zuid-Holland in the very early morning before dawn and can be observed again when after sunrise they pass by Noord-Holland. From population (est imates and migration data in the literature it is deduced that some 3500-6000) birds could pass by the Dutch coast, originating from the breeding populations N and E of the Netherlands. A simple extrapolation, not accounting for weather circumstances or daily rhythm suggests that in 1984 5500 birds migrated N along Noord-Holland, which would be a rather substantial proportion of the possible number. Springs with less frequent occurrence of the favourable wind directions, like for example 1977 with the extrapolation resulting in a mere 2500 birds, would show significantly less migrants.
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