Why is reduced male parental assistance detrimental to the reproductive success of secondary female house wrens?

Abstract

Abstract. Polygynous male house wrens, Troglodytes aedon, usually provide their second of two mates with little or no aid in feeding nestlings. Most 'poorly aided' secondary females raise fewer and lower 'quality' offspring than 'fully aided' females. The parental behaviour of poorly aided secondary females was examined to determine why reduced male parental assistance so often lowers their reproductive success. Compared with fully aided first-mated females, poorly aided secondary females spent less time in nests brooding nestlings (∼3 min/h), and increased their rate of prey delivery to nestlings. However, nestlings of poorly aided secondary females still received significantly fewer prey than nestlings of fully aided first-mated females during the first half of the nestling stage. To match the per caput rate at which pairs delivered prey at nests of fully aided first-mated females during this period, it was estimated that poorly aided secondary females would have to reduce time spent brooding nestlings to less than 50% of that shown by fully aided females. Mean metabolic rate of nestlings may be so reduced at this level of brooding that they could not ingest and metabolize all food received. Nevertheless, a review of the literature indicates that poorly aided secondary female house wrens clearly shift relatively less time from brooding to foraging than do poorly aided females in other small passerine bird species. In this paper, it is argued that maintenance of extensive brooding by poorly aided female wrens cannot be explained by a high risk of nestling death from hypothermia. Rather, females in this species may stay in nests to prevent removal of offspring from nests by conspecific intruders.