CAMPHUYSEN CJ & DERKS PJT (1989) Occurrence and mortality of Great Crested Grebes Podiceps cristatus along the Dutch coast, 1974 - 1986. LIMOSA 62 (2): 57-62.

The occurrence of Great Crested Grebes in Dutch coastal waters is described, using seawatching data, collected by the Club van Zeetrekwaarnemers (CvZ) during 1974-86, and results of beached bird surveys carried out in these years by the Nederlands Stookolieslachtoffer-Onderzoek (NSO). Numbers seen along the coast or found on the tide line were compared with weather circumstances, particularly the number of frost days (mean temperature below 0 0 C), and the number of days with ice covers over 50% on the IJsselmeer. The numbers of grebes using coastal habitats were found to increase. Numbers seen per hour at coastal sites along the mainland of Noord-Holland and Zuid-Holland in winter increased significantly. There was also a relation between numbers seen at these sites and the numbers of frost days. The largest number seen was 10 300 Great Crested Grebes heading south on 8 January 1985, Scheveningen (Zuid-Holland). Huge concentrations, numbering up to 5000 birds, were seen, but such flocks usually did not stay long in the coastal waters. Flocks of several hundreds, or perhaps a few thousands, stay for longer periods in these waters during winter. The numbers of corpses found at the tide line was closely correlated with the occurrence of severe winter conditions (fig. I). Few corpses have been dissected, but our data suggest that most grebes died from starvation and many were immatures or juveniles. Densities were low in 1986 and 1987 when many frost days occurred (fig. 2). Since most Great Crested Grebes found dead must have been immatures or juveniles (as our data suggest), the small figure in 1986 could be explained by recruitment failure. Very small numbers of juveniles were observed during autumn 1985 in the huge moulting flocks of grebes Great Crested Grebe Podiceps cristatus on the IJsselmeer (piersma 1987). Another aspect may be the lack of experience in grebes which are facing a severe winter the first time. In such a case, extensive mortality can only be expected in a severe winter following a series of mild winters.

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