Fifty : Fifty Offspring Sex Ratios in Savannah Sparrows

that natural selection should favor the ability of animals to modify the sex ratio of their offspring whenever the net fi tness benefi ts of producing sons or daughters vary as a function of parental condition. For example, when males compete strongly for females and have greater variance in reproductive success than females, parents in good condition should invest more in sons. Parents in poor condition, on the other hand, should invest more in daughters because they are unlikely to produce sons that would be successful in male–male competition. By focusing on offspring sex-ratio manipulation by individuals of different quality, Trivers and Willard (1973) extended work by Fisher (1930) who sought to explain overall 50:50 sex ratios in natural populations, which he hypothesized was driven by the advantage of investing in offspring of the rarer sex. Inspired by Fisher (1930) and Trivers and Willard (1973), numerous researchers have investigated offspring sex ratios in a variety of bird species, with mixed results (reviews in Clutton-Brock 1986, Gowaty 1991, Sheldon 1998). Many of the best examples of adaptively skewed offspring sex ratios involve birds maintained in captivity (Clotfelter 1996, Kilner 1998) or provisioned with artifi cial nest boxes (Ellegren et al. 1996, Svensson and Nilsson 1996, Bradbury et al. 1997, Kolliker et al. 1999, Albrecht 2000, Oddie 2000, Whittingham and Dunn 2000). Other studies supporting the Trivers-Willard model focus on birds with particular ecological characteristics: species that occupy habitats that are highly variable in quality (Cooch et al. 1997, Komdeur et al. 1997, Korpimaki et al. 2000), species that are highly polygynous or strongly sexually dimorphic in size (Dijkstra et al. 1998, Westerdahl et al. 2000), or species that breed cooperatively (Gowaty and Lennartz 1985, Komdeur et al. 1997). 1 E-mail: [email protected] ABSTRACT.—Over a 14 year period, we determined offspring sex ratios in a population of Savannah Sparrows (Passerculus sandwichensis) breeding on Kent Island, an isolated 80 ha island in the Bay of Fundy, New Brunswick, Canada, based on morphological measurements of 318 independent juveniles and 361 returning adults of known parentage. The mean annual offspring sex ratio was exactly 1.00 (±0.23, range= 0.61–1.41, n = 14). In no year did offspring sex ratio deviate signifi cantly from 50:50, regardless of the sex ratio of breeding adults. Offspring sex ratio did not vary as a function of the timing of breeding, maternal or paternal age, or parental mating status (monogamy vs. polygamy). Overall, our data provide strong support for 50:50 offspring sex ratios at the population level (Fisher 1930), but no support for the notion of adaptive modifi cation of offspring sex ratios by individuals (Trivers and Willard 1973). Received 25 July 2001, accepted 12 September 2002.