Behavioural studies of Great Northern Divers Gavia immer wintering on the coast of Jæren, SW Norway in 1996–2010, revealed interspecific aggression by divers against several species of waterbirds. Aggression was shown against two species of cormorants, five species of diving ducks, two species of grebes, one species of diver, one species of alcid, one species of merganser, and against Shelduck Tadorna tadorna. In relation to their occurrence in the vicinity of individual Great Northern Divers, piscivorous birds were significantly more often subject to aggression than other waterbirds. Aggression towards piscivorous species was in accordance with predictions from a food competition hypothesis, but possible benefits from threatening and attacking non-piscivorous ducks seem unclear. No support was found for hypotheses related to sexual selection or predation.
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