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Some ecological aspects of social behaviour in the song sparrow, Melospiza melodia Knapton, Richard Walter

Abstract

The purpose of this study is to investigate experimentally some ecological aspects of social behaviour in Song Sparrows, Melospiza melodia, on Reifel Island, British Columbia. Two main questions are asked: A. Does the temporal pattern of settling on territories influence the number of occupied territories, and hence territory size, and the breeding density in a given area? B. Are the juveniles that obtain territories, when the opportunity arises, the dominant individuals in dominance hierarchies established during the pre-breeding season? A necessary pre-requisite for both questions to be answered is that there exists a surplus of birds which are non-territorial and potentially capable of breeding, but which are prevented from taking territories by the resident territory holders. This also is experimentally investigated. Removal experiments were carried but in the fall of 1972 and the spring of 1973. Subsequent replacements and breeding showed that there was a surplus of Song Sparrows on the study area that were physiologically capable of breeding. All but one of the replacement birds were juveniles, and all were probably of local origin. Two types of removal experiments, simultaneous and successive, were carried out in both the spring and fall. On the Simultaneous Removal areas, total replacement took about nine to ten days. Significantly more territories were taken, and the increases in both spring and fall were about 40%. Further, the mean territory size after the removals was significantly smaller than that before. Finally, territory boundaries were completely rearranged following the removals. The replacement on the Successive Removal areas took upto three to four days. There was no significant difference in the number of territories taken, nor in the mean territory size, after the removals. Further, the territorial pattern was retained. Breeding density on all areas, however, remained much the same before and after the experiments. Therefore, there were several unmated males with territories after the removals, and these unmated males proved to have significantly smaller territories than mated ones. Factors which could have accounted for the different results of the simultaneous and the successive removal experiments are discussed, and a proposed explanation is given. Dominance hierarchies were determined in the loose groups of juvenile Song Sparrows that congregated over the winter at certain localities along the hedgerows. Each group tended to be a discrete unit, although some interchange of individuals (both dominant and subordinate) occurred. The hierarchies themselves were stable and essentially linear, with few reversals and the occasional triangle. The removal experiments presented an opportunity for some of the members of the hierarchies to obtain territories. It was found that dominant males in the hierarchies were the successful ones in establishing territories. Further, in the largest hierarchy, out of 12 juvenile males, 4 from the top 5 obtained territories. Factors which could possibly influence the position of the bird in the hierarchy are discussed, and the possible outcomes of the hierarchy are considered.

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