Territory acquisition and loss in male song sparrows

Abstract

Male song sparrows, Melospiza melodia, contested for territories year-round in a non-migratory population. Over 5 years, most settlers were yearlings, but many were older floaters that had previously owned a territory. Most turnovers occurred when floaters evicted territory owners from all or part of their territories. This finding contrasts with the assumptions of most models of dispersal and habitat settlement, i.e. that owners are always dominant to floaters, and that floaters gain territories only after an owner's death. Territory owners often evicted neighbours in the breeding period, when they were also likely to gain access to additional females. Floaters usually gained territories just prior to breeding and as breeding began. These patterns are consistent with the idea that male territory defence primarily serves to provide access to females. Middle-aged males more often expanded their territories and regained them after being evicted than 1- or 4-year-old males. Birds that floated as yearlings were more likely to float after owning a territory as an adult than were birds that gained a territory before April of their first breeding season. Individual differences in competitive ability persist throughout life, but vary with age. Factors that affect patterns of aggressive versus passive territory acquisition between populations include: migratory tendency, habitat saturation, length of the breeding period, the potential to breed outside the natal population, the relative survival rates of territory owners versus floaters, and the mortality rate of territory owners in the absence of challenges by floaters. An understanding of the territorial system is crucial to predicting patterns of dispersal and habitat distribution.