Swift parrot data including the sex of offspring and their hatch order

Most species produce equal numbers of sons and daughters, and sex differences in survival after parental care do not usually affect this pattern. Temporary overproduction of the scarcer sex can be adaptive when generations overlap, the sexes differ in life history expectations, and parents can anticipate future mating opportunities. However an alternative strategy of maximising the competitiveness of the more abundant sex in these circumstances remains unexplored. We develop theory showing how mothers can maximise reproductive value when future mate competition will be high by producing more sons in the advantageous early hatching positions within their broods. Our model for optimal birth order was supported by long term data of offspring sex in a parrot facing catastrophic female mortality caused by introduced predators. Swift parrots (Lathamus discolor) suffer high female mortality due to introduced sugar gliders (Petaurus breviceps) creating fluctuating male biased adult sex ratios. ...