Some species in the family of grebes (Podicipedidae) are gregarious and nest in extensive colonies, others are solitary breeders aggressively defending their territories. Others, including the Great Crested Grebe (Podiceps cristatus), can breed in colonies or as solitary pairs, suggesting that individuals are able to change from a solitary strategy to a more social pattern. This paper concentrates on observations during colony establishment and nesting in a colony of Great Crested Grebes near Lake Ijssel in the Netherlands. The arrival of the grebes at the colony and their egg laying was spread over a long period. From the start, the breeding birds tended to use the entire area of the vegetation available for building nest platforms and this led to a progressive reduction in the distances between neighboring pairs. The final nesting density was extremely high, with 14.3 nest platforms per 100 m2. On average, each nest held 3.75 eggs. The crowded nesting of the Great Crested Grebe was imposed upon the birds because of the absence of sufficient suitable nesting habitat. It is suggested that the extent of aggregation is dictated by the quality of the site and the ability of the grebes to reduce their aggressiveness towards one another. Social behavior did not primarily contribute to the breeding aggregation.
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Vol. 28 • No. 3