Complex sex allocation in the laughing kookaburra

Abstract

In groups of the cooperatively breeding laughing kookaburra (Dacelo novaeguineae), offspring sex varied with the type of social group and with hatch rank. Groups with female helpers, especially if all helpers were female, had male-biased clutch and fledging sex ratios. Groups without female helpers (unassisted pairs or male-only helpers) had female-biased clutch and fledging sex ratios. Breeding females responded facultatively to increases in the number of female helpers in their group by producing more male eggs. These biases may occur if breeding females try to limit the number of daughters recruited into their group because unlike male helpers, female helpers depress the breeding success of their parents. Across all nests, two-thirds of first-hatched young were male, two-thirds of second-hatched young were female, and the sex ratio of third-hatched young was even. Hatch rank sex ratios also varied dramatically between different types of social groups, from 16.7% for second-hatched nestlings of unassisted pairs to 100% for first-hatched nestlings of groups with only female helpers. A corollary of the relationship between hatch rank and sex was that hatching sex sequences were distributed nonrandomly: all groups avoided hatching a daughter first followed by a son (FM). Sibling competition is aggressive and sometimes fatal. Since females grow to be 15% larger than males the hatching sequence of sexes could affect nestling growth and mortality. However, an exhaustive analysis found little evidence that growth or survival of males was compromised if hatched after a sister. The small number of FM sequences may only have occurred in nests that were able to ameliorate any negative consequences. Alternatively, when clutch size is small and fledging success unpredictable because of brood reduction, the preferred brood sex ratio may be contingent on the number of fledged young, making it advantageous to order the sexes in the brood.