Female intrasexual reproductive competition in the facultatively polygynous song sparrow

Reproductive competition among females is an under-studied aspect of behavioural ecology. In species where males provide non-sharable resources that enhance individual and offspring fitness, such as feeding young, intrasexual conflict among females should be expected. My thesis examined reproductive competition among female song sparrows by estimating the reproductive costs of losing male care and behavioural strategies females employed to avoid the loss of male care. I used a long-term study of song sparrows, a facultatively polygynous passerine, on Mandarte Island, British Columbia Canada, to examine the potential reproductive and survival costs that polygyny might have on females. I found that polygynous females without male care experienced lower nest and lifetime reproductive success than polygynous females with male care. In contrast, female status within polygynous groups had no impact on overwinter survival. Three strategies that females might use to avoid polygyny or ensure access to male parental care while in polygyny include 1) intrasexual aggression to deter secondary females from settling, 2) infanticide of primary female’s nest by secondary females to improve nesting status or 3) nest timing to either increase the comparative worth of the nest through synchrony, or eliminate competition for male care through asynchrony. Using a mount presentation experiment I found that resident females reacted as predicted if intrasexual aggressive behaviour was used to deter secondary female settlement and ensure male parental care. Over 18 years when polygyny occurred in the population, I found evidence that the presence of secondary females was correlated with a rise in the nest failure rate of primary females, but I found no evidence that polygynous females used nest timing strategies to influence access to male care. Overall, my results suggest that female song sparrows use aggressive behaviours to reduce secondary female settlement, and within polygynous groups secondary females may use infanticide to advance their status. Despite the existence of female strategies to circumvent the loss of fitness due to polygynous mating, polygyny still occurred regularly in the population. This observation suggests that the strategies described above are often not effective, or that their costs outweigh the potential gains to individual fitness.

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