In late August 2001, I discovered an apparent communal roost of Black-throated Divers Gavia arctica at a loch in North Uist, Western Isles. Black-throated Divers are sociable birds, frequently gathering in flocks during the breeding season (Lehtonen 1970; Sjölander 1978; D. Jackson in litt.). Flocking can occur daily throughout the breeding season, with birds converging on recognised flocking are

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