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The kakapo is widely regarded as the world's most absurd bird. It is a flightless, night-active parrot that lives in New Zealand. It is thus a prime example of the sort of evolutionary experiment that happens on islands that lack serious predators.
Now, sadly, the kakapo population has been reduced to a few dozen individuals. But efforts to preserve the species have led to another sort of experiment—one that has produced the best evidence so far for a so called sex-allocation theory.
Bruce Robertson, of the University of Canterbury, and his colleagues, have been studying a mystery. To keep the kakapo population going, conservationists have been feeding the birds—in particular, the females—so that they stay above the minimum weight they need to breed. That worked splendidly as far as it went. Between 1997 and 2005 the adult population rose from 50 to 83. But the sex ratio went haywire. Those 83 birds are divided into 45 males and 38 females. Moreover, females were producing twice as many sons as daughters. Since another of the kakapo's peculiarities is its breeding system in which the male plays no part in raising the young, the survival prospects of the species are not enhanced by this biased sex ratio.
Dr. Robertson suspected that the sex ratio was not an accident. In all species the number of descendants left by individual males is more variable than the number left by individual females. That means your daughters are more likely to have at least some children than your sons, but successful sons have many more children than successful daughters. In a species like the kakapo, where the males have to display to the females in a fashion show, and where only the showiest males get picked to reproduce, it makes sense to have sons only if they are likely to be the best males around. Other wise it is better to have daughters.