UBC Theses and Dissertations
Intrasexual competition, dispersal, territoriality and the mating system of the song sparrow on Mandarte Island, B.C. Arcese, Peter
I consider the relationship between natal dispersal, the acquisition of breeding resources and the proximate maintenance of the mating system in a population of song sparrows (Melospiza melodia) resident on Mandarte Island, B.C.. The general hypothesis tested was that intrasexual competition for the resources that limit reproduction in each sex is the main factor affecting patterns of natal dispersal, territory turnover and mating system organization. This study reveals differences in the competitive ability of individuals and the consequences of these for natal dispersal, territory acquisition and the ability to monopolize mates. In this population, natal dispersal, territoriality and the mating system are related through the common mechanism of intrasexual competition for breeding resources. This competition affected reproduction within each sex, and in the opposite sex, raising potential conflicts of interest between mated males and females. I identify correlates of competitive ability, and consider how individuals of each sex maximize the number of young they raised. All suitable habitat was defended even at low density, resulting in high levels of competition for space. Song sparrows contested for territories year-round. Most settlers were yearlings, but several males were older and had previously owned a territory. Most floaters settled by evicting owners from all or part of their territories, in contrast to the assumption of most models of dispersal that floaters gain territories only after an owner's death. Relative age and dominance within cohorts affected recruitment and territorial status in the year following hatch in each sex. Differences in age also affected territorial and mating behaviour, particularly in males. Although competitive ability varied with age, differences in competitive ability between males persisted through life. Approximately 9% of male breeding attempts occurred after a male territory owner had evicted a neighbour and thereby gained access to an additional female. Polygynous males raised more young than monogamous males. Females vigorously defended their territories against female intruders, presumably to prevent polygyny by their mates. Females in polygynous groups often lost the aid of their mate, and raised fewer young, because polygynous males did not aid two females simultaneously. Adding supplemental food altered female time budgets, increased their ability to prevent settlement by female floaters, and increased their reproductive success. This suggests that females are constrained in the amount of time they can spend in territory defence by time spent foraging. The maintenance of monogamy was affected by differences in the abilities of individual males and females to defend their territories against intruders of the same sex. Dispersal distances were similar among sexes, in contrast to the usual pattern of female-biased dispersal in birds. Given the intense territorial behaviour of males and females, this study supports the hypothesis that biases in dispersal arise when territory establishment is substantially more difficult in one sex than the other. Dispersal on Mandarte was unrelated to inbreeding, common parentage, or reproductive success. However, emigrants from Mandarte were socially subordinate birds that must have outbred if they settled successfully. Female emigrants from local populations in the vicinity of Mandarte may settle more often than males. Dispersal occurred as young birds established ranges in the period following independence, from which they challenged or replaced owners. Adding food during the breeding period reduced dispersal in males and females in late summer and the following spring. Competitive ability probably affects dispersal from the natal population, the likelihood of settlement, and the time of settlement for those birds that remain in the natal population.