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Data of platform initiation, clutch initiation and clutch size of colonial Great Crested Grebes Podiceps cristatus were collected over a period of four years and analyzed with respect to seasonal patterns and possible differences between experienced birds and first-time breeders. Extraordinary large (above 5 eggs) and small (below 3 eggs) clutch sizes were analyzed separately and possible cases of conspecific brood parasitism were documented. Average full clutch sizes of Great Crested Grebes were seasonally decreasing and it was concluded that this should be the norm. The seasonal decline in final nest content was triggered by seasonally increasing numbers of small clutches and in parallel decreasing numbers of exceptionally large clutches. Also, parasitic egg reception increased average clutch size more in early than in later clutches. Brood parasitism was a major component in exceptionally large clutches. At least for experienced birds, the not parasitized clutch size should be comprised of 3 to 5 eggs. Differences in nesting density inside the colony could possibly explain part of the parasitic egg laying. Adverse weather conditions leading to the destruction of part of the platforms were found to play an important role, too. The breeding parameters associated with small clutches pointed at a high likelihood that these could have originated from inexperienced females that could have laid later in the season and produced fewer eggs.

Keywords: brood parasitism, clumping of nests, clutch initiation, clutch size, colonial breeding, experience, platform initiation, Podiceps cristatus.