SEX RATIO CONSTANCY IN THE RED-WINGED BLACKBIRD.

Abstract

The mean primary sex ratio (proportion of zygotes that are male) in most organisms with genetic sex determination is generally found to be very near 50%. Although phenotypic variance about the mean is generally small and masked by the large sampling variance due to the binomial distribution of the sexes (Edwards, 1958, 1970), the existence of loci modifying the sex ratio has been demonstrated in some organisms (Weir, 1953; Feinberg and Pimentel, 1966; Myers and Krebs, 1971; Beckenbach, 1978; others reviewed by Edwards, 1962a), and patterns of variation in the sex ratio of other organisms have occasionally been noted (see Parkes, 1926, for an early review). Trivers and Willard (1973), reviewing more recent mammalian data, proposed that facultative sex ratio variation is adaptive and is compatible with Fisher's (1958) hypothesis of evolution of the mean sex ratio. On the assumptions that (1) condition of young at the end of parental care will tend to be correlated with the condition of the mother during parental care, (2) differences in condition of the young at the end of parental care will tend to endure into adulthood, and (3) in polygynous species adult males are more greatly benefited by slight advantages in condition than are adult females, they hypothesized that mothers should bias their sex ratio in favor of males if they are in better than average condition for their population, and in favor of females if they are in poorer than average condition. Although Trivers and Willard considered only maternal condition, the logic of the argument applies to