Focal-animal studies of Great Northern Divers Gavia immer wintering on the coast of Jæren, SW Norway, showed social behaviour to vary over the season. Groups consisting of two adults and one or two juveniles arrived in autumn. The adults of such groups were aggressive toward neighbouring divers including other adults. In early winter juveniles as well as adults adopted a solitary and dispersed existence and adults showed aggressiveness also toward juveniles. Second winter birds were solitary throughout the winter, and they were intermediate between adults and juveniles in dominance level. Juveniles associated with two adults spent more time feeding and less time in comfort activities than the accompanying adults. The social groups containing adults and juveniles are interpreted as family groups in which parents perform prolonged parental care presumably allowing juveniles to gain experience with marine food and habitat. Some twosome adults without juvenile company are seen after arrival in autumn, and again in late winter some time before departure, suggesting that pair members might accompany one another on migration. Differences in social behaviour of birds wintering in Europe and in North America are briefly discussed.
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