Familiarity between mates improves few aspects of reproductive performance in house sparrows

Many hypotheses have been proposed to account for the increased reproductive success with age documented in most birds. One, the mate familiarity hypothesis, suggests that older individuals are likely to have partners with whom they have bred before, which may enhance reproductive success. We found that pairs of house sparrows that bred together previously fledged more young than pairs that were newly formed. This was primarily due to newly-formed pairs with at least one yearling having fewer nesting attempts than continuing pairs. We also examined the possibility that continuing pairs were better coordinated in parental duties, but found no evidence for this in terms of amount of overlap at the nestbox or amount of time nestlings were unattended. Our results suggest the higher success of continuing pairs may be a matter of female age and quality and not due to benefits accrued by coordination with their mate.