Mechanisms of Facultative Sex-Ratio Variation in Zebra Finches (Taeniopygia guttata)

Abstract

The offspring sex ratio at independence in a colony of Zebra Finches (Tae- niopygia guttata) was significantly male biased. Three possible proximate causes of this bias were examined. (1) The first was that there is a sex-biased laying sequence followed by brood reduction. Males were significantly more likely to hatch from earlier-laid eggs than females, but there was no evidence of increased mortality among late-laid nestlings. (2) The second was that there is sex-biased mortality due to sex-specific growth rates. Male and female offspring had similar rates of mass increase, tarsus growth, and wing-chord growth. (3) The third was that there is sex-biased provisioning by one or both parents, increasing survival probabilities of preferred sex. There was evidence that males fed male-biased broods more frequently than female-biased broods. Fledging age, fledging mass, and the number of days to independence were similar for male and female offspring, suggesting that sex-biased provisioning had no significant biological effect. Although none of the hypotheses adequately explained the observed sex-ratio bias, results from this and other studies suggest that sex- biased laying sequences followed by brood reduction are most likely to cause skewed offspring sex ratios in nature. The implications of a sex-biased laying sequence and of sex-biased provisioning by the male are discussed in reference to the potential evolutionary conflict between males and females over control of offspring sex ratios. Received 5 October 1994, accepted 21 June 1995.