DNA fingerprinting of 183 broods of house sparrows and their attendant adults revealed that 13.6% of nestlings were unrelated to the attendant male. The occurrence of cuckoldry was unaffected by male age and experience, pair bond duration or time within a breeding season. However, cuckoldry was significantly more frequent in broods that included some infertile eggs. This study suggests that sperm competition can be strongly influenced by the fertility of the cuckolded male, and that females may benefit from extra-pair copulation as an insurance against their mate's infertility.
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