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EGG incubation temperature can affect the sex ratio of birds – at least in one Australian species.

Temperature-dependent sex determination is well known in reptiles, but most birds’ sex is thought to be determined at fertilisation, before incubation begins. Ann Göth of Maquarie University, Sydney, decided to investigate after hearing reports from Aboriginal elders that the number of male and female Australian brush turkeys (Alectura lathami) differs according to whether the nesting season was hot or cold.

Brush turkeys belong to a group of birds called megapodes, which, like reptiles, use external heat sources to incubate their eggs. They cover the eggs with a mound of organic matter, which releases heat as it decomposes. Males control the temperature of the mound by adding or removing material daily.

The average incubation temperature in a mound is 34 °C. With colleague David Booth at the University of Queensland, Göth removed eggs from mounds and incubated them at either 31 °C, 34 °C or 36 °C. At 34 °C equal numbers of males and females hatched. But significantly more males hatched at 31 °C, and more females at 36 °C (Biology Letters, DOI: 10.1098/rsbl.2004.0247).

It is not clear why, but male embryos could be more likely to die at higher temperatures, and females at lower. However, recent research on certain fish, salamanders and lizards has shown that incubation temperatures can override the influence of sex chromosomes, Göth says.