Female-Biased Sex Ratio at Hatching in the Green Woodhoopoe

In a population of Green Woodhoopoes (Phoeniculus purpureus) in the Rift Valley of Kenya, food abundance and breeding success varied greatly as a result of highly unpredictable patterns and abundance of rainfall. A given pair of woodhoopoes may breed successfully from zero to three times between May and December. We found a female-biased sex ratio in first nests of the year among those female breeders that had few (0-2) helpers. Apart from predation of entire broods, nestling mortality was extremely low (5%), and the bias was not based on differential starvation of male and female chicks. We examined several possible explanations for the female-biased sex ratio, and we suggest that production of female offspring by breeders in small groups is adaptive in that females are smaller and probably less expensive to rear than males. Young female nest helpers contributed significantly more feedings to subsequent broods than did their male siblings of the same age. The bias toward daughters could be effected either via nonrandom segregation of sex chromosomes or by differential mortality of eggs by sex. In several other bird species, larger eggs may give rise to males. This provides a possible means for the female parent to discriminate the sex of offspring before hatching.