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UBC Theses and Dissertations

Female aggressiveness, breeding density, and monogamy in willow ptarmigan Hannon, Susan Jean


In this thesis I investigate the influence of aggression by females in setting breeding density and maintaining monogamy in a population of willow ptarmigan (Lagopus lagopus). The aims of the study were: 1) to test the hypothesis that female willow ptarmigan determine their own breeding density by spacing behaviour, independently of male density; 2) to evaluate the effect of interactions between the sexes on final breeding density; and 3) to examine factors which may constrain willow ptarmigan populations to monogamy. Sex ratio of the population was altered in spring by continuous removal of most males and females from separate plots. The effect of removals on numbers of the same and the opposite sex was monitored. The following results were obtained and conclusions reached: 1) Females and males defend territories against individuals of the same sex, and this behaviour prevents some potential recruits from breeding. Physiologically mature yearling females and males settled in response to the removal of territorial birds of like sex. 2) Density of territorial males may not determine the number of females that breed. Females settled at high density, despite a reduction in the density of males, defended territories against each other within the enlarged territories of the remaining males, and mated polygynously. 3) Settlement patterns and subsequent territory sizes of males may affect density of both males and females. Females preferred territories of medium to large size, and males with smaller territories often remained unmated. Competition by yearling males for limited space on the breeding area may reduce territory size below that acceptable to females. Females may also alter settlement patterns of males by ignoring territorial boundaries of males and inciting interactions between neighboring cocks. 4) Unshared male vigilance is not essential to, but may improve, female reproductive success in years of high predation. Polygynous females had similar breeding success and survived as well as monogamous females, except in a year of high nest predation when they suffered higher nest loss. 5) The aggressive behaviour of females may prevent polygyny from occurring in unmanipulated populations, if a polygyny threshold exists. Females are able to defend territories which are similar in size to those of males and thus can prevent secondary females from settling. Results of this study indicate that aggressive behaviour of females in a monogamous species may be an important factor in regulating population density. Future studies should examine physiological and ecological factors influencing agonistic behaviour of females and should attempt to manipulate female aggressiveness to test whether changes in this behaviour can cause changes in population density.

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