Effects of Brood-Size Manipulations on Parental Investment, Breeding Success, and Reproductive Endocrinology of House Sparrows

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Abstract
Brood sizes of House Sparrows (Passer domesticus) were altered experimentally by adding or subtracting 2 nestlings. Unaltered broods served as controls, and experimental brood sizes were within the normal range found under natural conditions. Feeding rates of both parents increased with brood size, and although nestling mass decreased with brood size, most pairs were able to fledge the extra young added to their broods. Males rearing larger broods invested less in nest-site defense and mate-guarding activities, and females rearing larger broods took longer to initiate subsequent broods and produced smaller subsequent clutches. However, the productivity of the subsequent broods did not decrease. Adult survivorship was not affected by the brood manipulations. In males, circulating levels of dihydrotestosterone increased significantly with brood size. Levels of other hormones, including luteinizing hormone (LH), testosterone, estradiol-17.beta., and corticosterone (B), were not related to brood size in either sex, although in females LH and B titers tended to increase with brood size. Males feeding larger broods tended to have less body fat, but otherwise there was no relationship between brood size and body condition. These results suggest that adults tending larger broods were not unduly stressed by their extra efforts, at least when feeding nestlings. However, the increased interbrood interval and decreased subsequent clutch size associated with rearing larger broods may have resulted either from the increased energetic and nutrient demand on females after the young fledged or simply from the extra time required to rear the additional feedlings to independence.