SEX‐RATIO MANIPULATION IN COLOR‐BANDED POPULATIONS OF ZEBRA FINCHES

Abstract

Significant correlations were found between attractiveness of leg-band color (determined by preference tests [Burley et al., 1982]) and sex ratio of offspring in two long-term breeding experiments involving zebra finches. In both experiments, birds with attractive band colors produced more same-sex offspring, while birds with unattractive band colors produced more opposite-sex offspring. The results of these experiments are consistent with those of a previous experiment (Burley, 1981). To explain the earlier results, I hypothesized that parents adjust their allocation to sons and daughters to produce offspring they "expect" to be most attractive. The purpose of such sex-ratio manipulation is to enhance fitness by the production of offspring with superior mate-getting opportunities. Two alternative hypotheses are presented here. One is that sex ratios change with parental age and/or experience. Evidence does not support this hypothesis. There were no temporal trends in sex ratio independent of band color. A second possibility is that sex ratios reflect differential parental ability to rear sons and daughters. This hypothesis cannot be conclusively tested on the basis of present evidence, but available evidence does not support it. Within color classes, weights of sons and daughters did not differ. Evidence indicates that parents effect secondary sex-ratio manipulation through the selective rejection of young, usually within six days of hatching. There is no evidence of manipulation prior to egg-laying. The costs associated with brood reduction probably set limits on the extent to which secondary manipulation can be profitably employed.