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We studied within-season mate switching in House Wrens (Troglodytes aedon) from 1998 to 2000. Males initiated mate switching, but mate switching was not related to the male’s paternity in the first brood, his body condition, or his breeding experience. Males were more likely to switch mates when unmated females were nearby. Males that switched mates sire a similar number of young as males that stayed with the same mate, and, thus, males did not appear to benefit directly from switching mates. In contrast, half of the females that were deserted did not find a second mate during the breeding season, and, thus, incurred a reproductive cost.