Parity in Fledging Sex Ratios in a Dimorphic Raptor, Montagu,s Harrier Circus pygargus

Fisher (1930) argued that the offspring sex ratio is the result of natural selection for equal investments in male and female offspring. If no other forces are acting, the overall offspring sex ratio in a population should be 1:1. Although the extraordinary sex ratios noted mainly in inverte-brates appear to be rare in birds (Krackow 1995), several evidences of sex ratio biases have been recorded during the past decade of so (Wiebe and Bortolotti 1992, Komdeur et al 1997, Torres and Drummond 1999). Biased sex ratios may be favored in species where the reproductive costs or fitness benefits vary between sons and daughters (Clutton-Brock et al. 1985, Weatherhead and Teather 1991, Post et al. 1999). Consequently sex ratios have been widely studied in sexually dimor-phic birds; the greater food requirements of the larger sex makes it more costly for parents to raise and could also render it more susceptible to food shortages, leading to increased mortality of chicks of the larger sex during periods of environmental stress (Clutton-Brock et al. 1985, Weatherhead and Teather 1991). Additionally circumstances at the time of breeding may affect the parental condi-tion, and parents might manipulate the sex of the progeny to lessen the risk of failure or maximiz