Importance of monogamous male birds in determining reproductive success

Summary Several recent studies have questioned whether the presence and activities of monogamous male birds are of value to their young during the nesting attempt. We addressed this issue in house wrens (Troglodytes aedon) by removing males early in the nestling stage. In one of four periods survivorship among experimental nestlings was 63% lower than among control nestlings. In the other periods survivorship was about equal in experimental and control nests indicating that in this species the male's activities may only benefit his nestlings during unfavorable periods. We summarize the results of 15 other male removal studies, concluding that when males appear to be helping, their absence usually results in decreased survival of young whereas when the males render little apparent care their absence does not decrease survival of the young. A possible exception to this generalization, illustrated by our study and others, is that during the nestling stage of many altricial species, the female appears able to raise the young about as well without the male as with him, except in poor periods. A male caring for his nestlings during good periods may benefit by maintaining his mate's health for the next nesting attempt or by insuring her willingness to mate with him again. Alternatively, it may cost males so little to help during good periods, and benefit them so much during poor periods, that there has been little selective pressure for them to assess conditions and vary their level of care accordingly. A series of more specific predictions about which males are likely to be of greatest, and least, value to their nestlings is presented. Male care after young leave the nest has not been well studied, but is probably substantial in many species, and warrants attention in future studies.