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Nightjars represent a model taxon for investigating physiological limits of heat tolerance because of their habit of roosting and nesting in sunlit sites during the heat of the day. We investigated the physiological responses of Rufous-cheeked nightjars (Caprimulgus rufigena) and Freckled nightjars (Caprimulgus tristigma) to high air temperatures (T _a) by measuring body temperature (T _b), resting metabolic rate (RMR) and total evaporative water loss (TEWL) at T _a ranging from 10 to 56 °C. Both species became hyperthermic at T _a > T _b. Lower critical limits of thermoneutrality occurred at T _a between 35 and 37 °C, whereas we detected no clear upper critical limits of thermoneutrality. Between T _a ≈ 37.0 and 39.9 °C, rates of TEWL increased rapidly with T _a. At T _a ≥ 40 °C, fractional …