Kleurcontrast in de vleugeldekveren bij Alk en Zeekoet

In the practice of ringing songbirds a wellknown character for age-determination is the contrast between old (juvenile) greater wingcoverts and new ones. Kuschert et.al. (1981) found this character to be applicable to first winter birds of Guillemot and Razorbill. As evidence they considered a.o. the 100% correlation be- tween bill-height and lack of grooves in first winter Razorbill and the occurrence of wing-contrast; another part of the evidence was given by the presence of contrast in 3 banded first winter Guillemots and the absence of it in an older banded bird. A second age criterium was found by Kuschert, namely the occurrence of white tips on the greater underwingcoverts in the Guillemot. Some 180 Guillemots and 78 Razorbills, found dead on the Netherlands coast in winter 1982/83, were examined on the two criteria apart from several other issues (see for these the papers of Van Franeker and of De Wijs). Figures of 49 young Guillemots and 9 young Razorbills were compared with the figures presented by Kuschert. The moult of the greater wingcoverts takes place from the body outwards, with the only exception that GWC (Dutch: ADV) nr.1 is moulted just before nr.2 (figure 7). In some birds, especially in the Razorbill (33%) all the greater wingcoverts are moulted. Then there is only a colourcontrast between the GWC and the primary coverts (PC). This contrast can be difficult to observe. In the Guillemot some renewal of the primary coverts takes place, especially nrs. 9 and 10 (figure 8). This is a very uncommon phenomenon, since most bird species moult these coverts from 1 to 10 outwards and each one just before the primary of the same number. In chapter 3.3 and figure 6 some attention is given to the differences in length of the GWC that accompany the difference in colour (contrast). These differences in length can be very helpfull to determine the exact limit of contrast. The author suggests a link between the relatively longer juvenile GWC1 and 2 and the unique behaviour of the fledgelings: they jump off the nesting-ridges at an age of approximately 18 days and reach the sea only on the “lifting power” of the wingcoverts as primaries and secondaries have not developed yet. Differences in wingcovert-moult between Guillemot and Razorbill (table 3) are peculiar and are not understood by the author. The most important ecological differences between both species is the bodyweight (Razorbill slightly more than half of Guillemot) and the timing of later moultcycles (Glutz von Blotzheim & Bauer 1982). They don’t seem to have anything to do with the moult of the wingcoverts. The characters of contrast and white tips on underwingcoverts can be used in fieldwork, but warnings are given. After a first inspection on the beach, one should give the wings a second glance when they are cleaned and dried at home.