Dispersion and feeding ecology of wintering cormorants ( Phalacrocorax carbo)
The dispersion of wintering cormorants (Phalacrocorax carbo) was studied in an estuarine habitat in northeast Scotland. Birds were dispersed in mobile clumps both when roosting and when foraging. A cormorant's choice of roosting site appeared to be influenced primarily by the nature of the site, but the use of decoys showed that groups arose on suitable sites by social attraction. There was evidence that the communal roost had survival value as an antipredator strategy. The use of decoys demonstrated that foraging clumps arose through social attraction. Cormorants also followed each other from the communal roost to the foraging grounds. It was concluded that the roost acts as an "information centre" in extending the benefits of social foraging by local enhancement in food-finding. Individual foraging success was not influenced significantly by tidal variations or water-roughness and associated turbidities, but success was higher around midday than at other times of day and most birds foraged around this time. Individual foraging success was not affected by the number of birds foraging but rose significantly with increasing clumping. Adult cormorants were more successful than juveniles and captured larger fish. Foraging in tight groups was thought to be more advantageous to the more inexperienced juvenile birds in reducing the risk of no success compared to foraging other than in groups or alone. The enhanced feeding success of birds in clumps was considered to be a consequence of groups arising in the best foraging areas through local enhancement rather than a result of birds in groups behaving differently from non-group foragers. The dispersion in loose flocks was seen as a strategy for successful exploitation of non-shoaling cryptic prey dispersed in unpredictably located patches.