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Konter A. 2011. Seasonal patterns of aggressiveness in colonial Great Crested Grebes Podiceps cristatus. Ardea 99: 85–92. In birds, coloniality is considered to be evolutionary more advanced than solitary breeding. Territorial Great Crested Grebes Podiceps cristatus may engage in opportunistic colonial breeding. The present study focuses on behavioural aspects associated with clumped nesting. Based on the assumption that different agonistic interactions may trigger different costs, aggressiveness during colony establishment and subsequent nesting in the Great Crested Grebe was measured. Upon platform initiation, focal pairs claimed a territory by performing intense platform courtship. With agonistic behaviour they tried to prevent additional conspecifics from settling in their direct vicinity. As a consequence, early levels of aggression in the entire colony increased with the numbers of settled pairs. Quite quickly however, aggressiveness declined despite additional pairs settling and long before the maximum number of platforms for a particular year was reached. Two main phenomena may have contributed to the decline in aggression. First, colonial Great Crested Grebes could have been conducting behaviour consistent with the ‘dear-enemy-effect’. Second, habituation to or accommodation with new neighbours could have been helped by behavioural adaptations. Focal pairs provided evidence for a relationship between aggression and egg-laying. The aggressive behaviour that was observed after clutch initiation could have been an effort to reduce the risk of intraspecific brood parasitism. Thereafter, fighting was uncommon and aggression was greatly limited to ritualized threatening. Aggression levels were higher in areas of high nesting density prior to and during egg-laying, but no longer so after clutch completion. It is concluded that colony establishment in the Great Crested Grebe is not an index of the species' sociability, but rather expresses its phenotypic plasticity to adapt to close neighbours and to contain its aggressiveness.

Seasonal Patterns of Aggressiveness in Colonial Great Crested GrebesPodiceps cristatus