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Extra-pair mating and effective population size in the song sparrow (Melospiza melodia) O’Connor, Kathleen D.


Effective population size is used widely in conservation research and management as an indicator of the genetic state of populations. However, estimates of effective population size for socially monogamous species can vary with the frequency of matings outside of the social pair. I investigated the effect of cryptic extra-pair fertilization on effective population size estimates using four years of demographic and genetic data from a resident population of song sparrows (Melospiza melodia Oberholser 1899) on Mandarte Island, British Columbia, Canada. Estimates of effective population size using genetic data were nearly identical to estimates based on social data, even though 57% of 84 females engaged in extra-pair matings and 28% of 471 young that survived to independence were from extra-pair matings. Estimates of variance in male reproductive success were also similar using genetic and social data. These results suggest that it is not necessary to determine the sires of offspring to estimate effective population size accurately in socially monogamous species where extra-pair mating occurs. The benefits to females from mating outside of the social pair are not fully understood. I tested the genetic compatibility hypothesis, i.e. that females chose extra-pair mates to avoid inbreeding. Using four years of genetic data, I assigned paternity to offspring that survived at least to six days of age. Females in a pair with a high kinship coefficient were not more likely to engage in matings outside of the pair bond. Females did not choose sires that were less related to them than their mate, nor did they did choose sires less related to them than the average kinship coefficient of candidate males in adjacent territories. Furthermore, extra-pair young did not survive better or produce more offspring than within-pair young. Although I could not rule out the possibility that extra-pair offspring hatch at higher frequencies or have better immune function than within-pair young, I found no support for the genetic compatibility hypothesis.

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