Secondary Sex Ratio in Anatinae

Abstract

Most waterfowl have highly skewed tertiary (adult) sex ratios in favor of males (Bellrose et al. 1961, Aldrich 1973, Bellrose 1980, Owen and Dix 1986). Sex ratios of diving ducks generally are more distorted than those of dabbling ducks (Mendall 1958, Aldrich 1973, Bellrose 1980). Perhaps the greatest disparity has been recorded for Canvasbacks (Aythya valisineria), and Common Pochards (Aythya ferina), with two to three or more males for every female in some major wintering areas (Haramis et al. 1985, Owen and Dix 1986). The skewed sex ratio among Temperate Zone ducks is of particular interest because most of these species show seasonal monogamy (reviewed by Rohwer and Anderson 1988, Oring and Sayler 1992). Thus, many males have little opportunity for reproduction, especially because the secondary reproductive tactic of forced copulation appears to be confined to successfully paired individuals (McKinney et al. 1983, McKinney 1985, Gauthier 1988). However, it is important to realize that male-biased sex ratios in adult ducks are the product, not the cause, of seasonal monogamy (Oring and Sayler 1992). The strong and consistent skew in tertiary sex ratios raises the question of when and how skewed sex ratios develop, and suggests that females may manipulate sex ratios. If females can control the sex ratio of her progeny to increase survival of one sex, parental fitness will increase too (Leblanc 1987). Surprisingly little information has been published on secondary (at hatching) sex ratios of ducks. Most data come from ducklings hatched in an incubator from eggs collected from wild free-ranging females and provide no evidence that sex ratios at hatching differ from unity (Sowls 1955, Mendall 1958, Bellrose et al. 1961, Swennen et al. 1979). Dubovsky (1990) hatched captive wild-strain and game-farm female Mallard (Anas platyrhynchos) eggs in an incubator and found no evidence that sex ratio in this species varies with laying order, egg mass, and clutch sequence within a breeding season. More complete information is available for other bird taxa, but the evidence for adaptive control of offspring sex is scantand controversial (reviewed by Clutton-Brock 1986, Ligon and Ligon 1990). For example, some studies report variation in sex ratios at hatching as a function of laying or hatching sequence