Abstract:

Some bird species have both colonial and solitary breeding behaviour. One such species is the Tree Sparrow (Passer montanus), which is suitable for studying the adaptive significance of coloniality and the reasons why alternative breeding tactics can be maintained as viable strategies. We simulated both colonial and solitary breeding situations with dense and sparse spacings of artificial nestboxes and focused on the breeding performance and the returning rate of Tree Sparrows favouring dense or sparse nesting situations. This paper is a synthesis of results published in Acta Oecologica 1993, 14: 447-487; Journal of Animal Ecology 1994, 63: 265-274; Behavioral Ecology and Sociobiology 1994, 34: 113-124. Evidence proved that colonial and solitary breeding may be maintained as viable reproductive strategies. It is possible for birds to choose different sociality on the basis of breeding experience and to attempt to improve their performance by changing nesting situation. Three principal factors seem to induce the retention of alternative breeding tactics as viable reproductive strategies. (1) Multibreeding. Species should be able to breed two or three times within a breeding season. (2) Food environment should become richer and/or more diverse as the season progresses. (There is an adaptive adjustment in Tree Sparrow to increase food supply: parents rear more fledglings in second and third broods than in first broods.) (3) Difference in breeding experience between young and old parents. Older parents which have acquired experience prefer solitary nesting behaviour.