Breeding Behaviour, Productivity and Survival of Colonial and Non-Colonial Shelducks Tadorna tadorna

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Abstract
The productivity, population dynamics and behavior are compared in shelducks breeding in colonial and dispersed situations in the Firth of Forth, southeast Scotland. Most pairs defend feeding sites from late winter onwards. A main function of territories is to provide a relatively undisturbed feeding area for incubating ducks. Territorial behavior appears to exclude some birds from the breeding population but, in colonies, does not impose an unvarying limit to population size, as territory size and degree of overlap are not fixed rigidly. In colonies, 0.04 to 0.32 young were fledged per year per territorial adult, compared with 0.72 to 1.22 in dispersed situations. The main source of the difference between breeding output from colonial and isolated sites was the higher (.times. 6) duckling survival at isolated sites, associated with earlier fledging and lack of creching. Losses at colonies were due chiefly to predation by herring gulls while parents with broods were fighting with other shelducks; this did not occur, or was rare, at isolated sites. The population at the Aberlady Bay colony was self-supporting in, at most, 1 yr since 1967, and was subject to large-scale immigration, but isolated sites probably had surplus production. Breeding behavior is probably adapted to dispersed breeding, and cannot adapt to the aggregations because these are not normally self-supporting and so lack genetic isolation.