The average incubation temperature in natural Australian brush-turkey incubation mounds is ca.
To further elucidate the ecological significance of the observed sex bias, we also report on how incubation temperature affects embryo mortality as well as the mass and size of hatchlings, both of which are likely to affect offspring fitness. Here, we report that incubation temperature does affect sex ratios of hatchling Australian brush-turkeys, megapodes that build incubation mounds of organic material in which incubation heat is produced by microbial decomposition. The Australian brush-turkey belongs to the family Megapodiidae, which is the only bird family that uses external heat sources for incubation-a strategy similar to that of reptiles. Based on this information, we tested the hypothesis that such a skewing of sex ratios could be caused by differences in incubation temperature. However, an Aboriginal elder told one of us (A.G.) that the number of male and female Australian brush-turkeys ( Alectura lathami) differs after hot and cold nesting seasons (W. There is, so far, no convincing evidence that incubation temperature can affect sex ratios of bird hatchlings ( Pike & Petrie 2002). Temperature-dependent sex determination (TSD) during incubation is a well-known phenomenon in reptiles, whereas birds have genotypic sex determination (GSD) in which sex is determined at fertilization long before the incubation of eggs begins ( Hardy 2002). Instead, our data suggest a sex-biased temperature-sensitive embryo mortality because mortality was greater at the lower and higher temperatures, and minimal at the middle temperature where the sex ratio was 1 : 1. Megapodes possess heteromorphic sex chromosomes like other birds, which eliminates temperature-dependent sex determination, as described for reptiles, as the mechanism behind the skewed sex ratios at high and low temperatures. Chicks from lower temperatures weigh less, which probably affects offspring survival, but are not smaller.
In the Australian brush-turkey Alectura lathami, a mound-building megapode, more males hatch at low incubation temperatures and more females hatch at high temperatures, whereas the proportion is 1 : 1 at the average temperature found in natural mounds. Here, we show that incubation temperature does affect sex ratios in megapodes, which are exceptional among birds because they use environmental heat sources for incubation. To our knowledge, there is, so far, no evidence that incubation temperature can affect sex ratios in birds, although this is common in reptiles.
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