In oviparous reptiles, incubation temperature has been shown to have profound effects on embryonic development and hatchling phenotypes. However, these effects are not well studied in birds, because they typically brood their eggs within a narrow range of temperatures. The Australian Brush-turkey (Alectura lathami) is a megapode that constructs incubation mounds and relies on the heat produced by respiring microorganisms in these mounds to incubate its eggs and is, therefore, a useful comparative model. We developed a new method for monitoring mound and egg temperature to determine both mound thermal variability and the thermal tolerance of embryos. All mounds exhibited greater temperature fluctuations than previously reported or predicted by modeling. Furthermore, all Australian Brush-turkey embryos were exposed to suboptimal temperatures for prolonged periods during development, some experiencing temperatures 6°C above or 9°C below the optimum (34°C) for 12 h. This is the first evidence of bird embryos developing during long-term exposure to suboptimal temperatures. Notably, natural incubation periods were 2–6 days shorter than previously reported for this species.