Credit: STEVEN DAVID MILLER/BBC

Most organisms produce roughly equal numbers of male and female offspring and, many years ago, R. A. Fisher proposed the explanation for this — in a population in which one sex predominates, the members of that sex will produce fewer offspring (on average) than will those of the minority sex. So, genotypes that produce more of the minority sex than the population average will be favoured.

There is increasing evidence that some female birds control the ratio of sons to daughters in their families1,2. The latest example is reported by Heinsenet al.3 in this month's Proceedings of the Royal Society. The authors found that most eclectus parrots (Eclectus roratus) produce long runs of offspring of one sex — one female produced 20 sons in succession, followed by a run of 13 daughters.

Avian sex-ratios may depend on the position of an egg in the laying sequence, the date of breeding, maternal age, and the sexual attractiveness and condition of the father2. Before the new study, the most extreme variation in sex allocation in birds was shown by the Seychelles warbler, Acrocephalus sechellensis4. In this species, young females often remain on their parents' territories and help to rear subsequent offspring (cooperative breeding), whereas young males usually move away. On high-quality territories, help from the daughters increases the parents' reproductive success, but on low-quality territories it is outweighed by the amount of resources that these helpers consume. So, daughters are a disadvantage on poor territories, and only 13 per cent of offspring produced on such territories are female, contrasting with 77 per cent on good territories.

The Seychelles' warbler story is relevant to the new study because — uniquely among parrots — E. roratus is also a cooperative breeder, with up to ten males associating with a single breeding female (or, sometimes, with one or two other females). Unfortunately, the natural history of the species is poorly known. Moreover, the sex-ratio results were based on the records of aviculturalists, who keep the birds in traditional pairs. These are so different from the natural breeding system that they do not provide clues as to what determines sex-ratio adjustment. One can only speculate that, in nature, there is an advantage to females in being able to adjust the sex of their offspring in relation to the relative numbers of males and females in the breeding group.