Physiologists have primarily focused on two potential explanations for heat stress in animals—the classic model of molecular stability and an alternative model of oxygen limitation. Although the classic model has widespread support, the oxygen-supply model applies to many aquatic animals and some terrestrial ones. In particular, the embryonic stage of terrestrial animals seems most susceptible to oxygen limitation because embryos acquire oxygen from the atmosphere by diffusion rather than ventilation. We report experiments confirming the two conditions of the oxygen-supply model in Japanese quail embryos, Coturnix coturnix . Hypoxia (12% O 2 ) greatly reduced the chance of survival at 47.5°C, and hyperoxia greatly improved the chance of survival at 48.5°C. This finding expands the scope of the oxygen-supply model to a terrestrial, endothermic species, suggesting that oxygen supply generally limits the heat tolerance of embryos.