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

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#, 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 fledglings to independence. Received 10 July 1986, accepted 1 March 1987. IMPLICIT in many discussions of life-history phenomena is the assumption that organisms trade off two or more conflicting activities in a manner that maximizes their overall reproductive output (e.g. Williams 1966, Charnov and Krebs 1974). For example, most iteroparous organisms are assumed to forgo maximum annual productivity to increase subsequent survivorship, thereby maximizing lifetime fecundity (e.g. Kluyver 1963, Williams 1966, Charnov and Krebs 1974). Moreover, even within a given breeding season, other trade-offs may occur (e.g. McGillivray 1983). For males, Trivers (1972) proposed a fundamental trade-off between competing with other males to fertilize the ova of females and providing parental care to the offspring they produce. Ecological restrictions on these two options have been used to explain mating systems (e.g. Emlen and Oring 1977). We have suggested that endocrine and associ1 Present address: Department of Zoology, NJ-15, University of Washington, Seattle, Washington 98195 USA. ated behavioral changes of monogamous male House Sparrows (Passer domesticus) during the breeding season reflect a compromise between these two options (Hegner and Wingfield 1986a). In our study area, a given pair of House Sparrows may attempt from 3 to 5 broods within a nesting season. In adults, circulating levels of reproductive hormones vary considerably during different stages of nesting. Plasma concentrations of luteinizing hormone (LH), androgens, and estrogens are maximum during the egg-laying stage of each brood, decline rapidly during incubation, remain low after the young hatch, and rise again as the young approach fledging. In females, rising levels of LH and estradiol (E2) following fledging are indications of physiological preparation for laying the next clutch (Hegner and Wingfield 1986b). In males, levels of LH and testosterone rise rapidly to maximum concentrations when nestlings reach 9-10 days of age, well before the young fledge (typically when 14-15 days old). This reflects a transition between a state of high investment in parental care, when testosterone levels are 470 The Auk 104: 470-480. July 1987 This content downloaded from 157.55.39.4 on Fri, 09 Sep 2016 04:28:40 UTC All use subject to http://about.jstor.org/terms Julv 19871 Brood Manipulations in House Sparrows 471 low, to one of high investment in male-male competition, when testosterone levels are high (Hegner and Wingfield 1986a). We examined the physiological and reproductive effects of altering the demand for parental care on free-living House Sparrows. For some pairs we increased the demand for parental care by adding two young to the nest, for others we decreased that demand by removing two young from the nest, and for a third group the number of nestlings was unchanged. We subsequently determined the effects of these manipulations on investment in parental care, investment by males in male-male competition, circulating levels of reproductive hormones, body condition, breeding success in the experimental and subsequent broods, and adult survival to the next year.