Sex Ratios of Zebra Finches

Abstract

Tertiary sex ratios of Zebra Finches Poephila guttata are weakly but consistently male-biased (52.1%) over a broad geographic range. The cause of this bias does not appear to be differential mortality of adults, because adult males and females remain resident at study sites for equal lengths of time. The estimated secondary sex ratio (49.2%), combined with evidence of higher (more male-biased) sex ratios of young buds that remain on their natal site past maturity vis-a-vis those that do not and vis-a-vis adult immigrants, suggest possible differential mortality and dispersal of young females. Secondary sex ratios fluctuate in timelspace. Long-term (1975-1986) data for northern Victoria indicate that the sex ratio is inversely proportional to the rainfall accumulated during the immediately preceding season. During 1986, the sex ratio declined over the breeding (dry) season in the Top End, and a strongly male-biased (59.2%) sex ratio was produced in Alice Springs in a breeding pulse that followed the end of a two-year drought. Virtually identical sex ratios were obtained by mistnetting and the use of baited traps.