Breeding density, synchrony, and experience are expected to influence the frequency of extrapair paternity in birds. Using DNA fingerprinting, we examined the effect of these factors on tree swallows nesting at relatively high (grids of nest-boxes) and low (solitary boxes at least 100 m from the nearest neighbor) densities and in relatively synchronous (Alberta) and asynchronous (Ontario) populations in Canada. The mean percentage of extrapair offspring per nest did not differ significantly between birds nesting in grids (43%, n = 22 families) and solitary boxes (57%, n = 12 families). Similarly, there was no significant difference in the mean percentage of extrapair offspring per nest between relatively synchronous (60%, n = 12 Alberta families) and asynchronous (41%, n = 22 Ontario families) populations. We also found no consistent pattern between extrapair paternity and breeding experience among seven males and seven females examined over two to three breeding seasons. Female tree swallows can influence the fertilization success of extrapair males by active selection and rejection of copulation partners. We suggest that this ability limits the predicted effect of various ecological factors on the frequency of extrapair paternity

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