Experimental evaluation of age, body size, and experience in determining territory ownership in willow ptarmigan

During 1981–1984, 18% of resident males in a population of willow ptarmigan at La Perouse Bay, Manitoba, Canada, were unpaired at onset of breeding, whereas all resident females acquired males. Territory ownership and pair bonds were very stable within seasons, with only three territory and mate takeovers by males occurring among 275 pair-years. I present data on origins, age, body size and mass, familiarity with the site, and breeding experience for paired and unpaired males, and on responses of both sexes to vacancies created by the experimental removal of mates. Of 61 paired males (territory owners) removed after onset of incubation, 70% were replaced by either unpaired males or, to a lesser extent, by paired territorial neighbours. When seven original owners were released after over a week in captivity, five regained their territories and mates, displacing replacement males quickly. Initial territorial dominance was related to age, body size, site familiarity, and breeding experience, as original paired owners were older, larger, and more likely than unpaired males to have resided on the area and to have been paired in a previous year. However, neither age, body size, body mass, familiarity with the area, nor territorial or breeding experience predicted which unpaired males obtained territories and mates when opportunities were created. In contrast to males, females killed during incubation were not replaced. In other populations of willow ptarmigan where removals were done about 1 month earlier in the breeding season, females replaced at vacancies and males that replaced were more likely to be territorial paired males expanding their boundaries than unpaired males.