Evolution of Clutch Size: An Experimental Test in the House Wren (Troglodytes aedon)

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Abstract
(1) We manipulated the brood size of house wrens (Troglodytes aedon Vieillot) to determine (i) which brood size, and hence clutch size, is the most productive, and (ii) if there is a cost of reproduction that increases as an individual's brood size increases. (2) Early and late-season broods of above-modal size produced more fledglings than did broods of modal size. Chicks raised in early-season broods of above-modal size weighed less than those in broods of below-modal size, but the difference is small (0.39 g, 4%). Weights of chicks raised in above-modal, modal and below-modal broods late in the season were similar to each other. (3) Enlarged broods produced as many fledglings as did normal broods of the same size. Weights of chicks from enlarged broods were not significantly different from those of chicks raised in normal broods of the same size. (4) Females that raised enlarged first broods were not less likely than other females to attempt a second brood on the study area; they also did not have a longer interbrood interval or produce smaller second clutches than did females that raised broods of normal size. Female weight during the nesting cycle was not related to size of brood or to treatment. (5) We conclude that house wrens can raise additional chicks, and that there is no evidence that clutch size is adjusted to a single population optimum or to the maximum number of high-quality fledglings that the parents can raise. Wrens appear to be withholding reproductive effort, yet we find no evidence that raising additional chicks produces adverse effects either on subsequent reproductive attempts in the same season or on female condition.