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. Offspring hatched early within broods fledged in better
condition, and in support of our model were more likely to be male in
years with higher adult female mortality. We found a highly significant
rank-order correlation between observed and predicted birth sex ratios.
Our study shows the potential for mothers to maximise reproductive value
via strategic biases in offspring sex depending on the advantages
conferred by birth order and the predictability of future mate
competition. Our long term data support the predictions and appear to
suggest that sex allocation strategies may evolve surprisingly quickly
when anthropogenic pressures on populations are severe.