Language: English, with trilingual introduction in English, German, and French
In a study over five years of aggregated breeding in a population of Great Crested Grebes Podiceps cristatus at the Compagnieshaven of Enkhuizen on the eutrophic Dutch IJsselmeer, it appeared that this phenomenon there was imposed upon the birds by the abundance of food coupled with a scarcity of nesting sites. The arrival and the incubation processes of the different pairs were rather asynchronous. From the start of settlement, the grebes used the entire habitat available and later arrivals had to insist to gain access to the site. This led each year to a gradual decrease of distances between neighbouring platforms ending up at minima of 20 cm only and averages of about 2 m. Other studies confirmed the resulting rather long lasting nesting process. However, in very small colonies or in mixed colonies with more aggressive species, the grebes could be more synchronous.
Clutch size at Enkhuizen displayed a seasonal decrease. This trend was only partially confirmed by other studies so that no simple relationship such as early occupancy of breeding sites by experienced birds laying bigger clutches and later settlement of inexperienced grebes laying smaller clutches appeared. In addition, egg loss and parasitic egg laying impacted clutch sizes, especially under adverse weather conditions. Different comparative studies available suggested that the breeding success of colonial grebes could be superior to the one of solitary pairs on the same lake so that aggregated nesting was beneficial.
Fierce territory defence by established pairs was detected as a main cause for the extended building up process of the colony in Enkhuizen. Ethological data provided evidence for a gradual habituation to close neighbours in settled grebes. Especially with clutch initiation, aggression levels fell.
Situating the coloniality of the Great Crested Grebe in a more general evolutionary process, it was found to agree rather well with early stages of aggregated nesting and the definition of the nominal colonial breeder of Siegel-Causey and Kharitonov (1990). Indeed, consequential features of aggregation that allow limiting the associated costs appear to be hardly developed.
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