VLUG JJ (1974) Concentrations in summer of Great Crested Grebes Podiceps cristatus in Friesland on the lake lJsselmeer. LIMOSA 47 (1): 16-22.

The author and P. J. Zomerdijk observed large summer concentrations of the Great Crested Grebe Podiceps cristatus on the lake IJsselmeer near the South-coast of the province of Friesland. The largest numbers were observed near the Mokkebank and its surroundings (Fig. 1). In the beginning of July the first birds arrive. The largest numbers are seen in August, September and the beginning of October (up till twenty-thousand Great Crested Grebes, which is more than the whole Dutch breeding population). In the second half of October all the: animals disappear. By taking samples of the whole concentration the author investigated the number of juveniles. This number never exceeded 25% of the whole concentration. The Gteat Crested Grebes moult near the Mokkebank, probably because this locality offers protection against the predominating westerly winds. The author presumes that the concentration consists mainly of grebes that lost their eggs or young and consequently left their breeding-grounds. Protection of the shallow Mokkebank area on the IJsse1meer coast of the Province of Friesland will be necessary as these birds are highly vulnerable to disturbances by human activities.

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