Many birds living in regions with seasonal fluctuations in ambient temperatures (T(a)) typically respond to cold by increasing insulation and adjusting metabolic rate. Seasonal variation in thermal physiology has not been studied for the Caprimulgiformes, an order of birds that generally have basal metabolic rates (BMR) lower than predicted for their body mass. We measured the metabolic rate and thermal conductance of Australian owlet-nightjars (Aegotheles cristatus) during summer and winter using open-flow respirometry. Within the thermoneutral zone (TNZ; 31.3 to 34.8 degrees C), there was no seasonal difference in BMR or thermal conductance (C), but body temperature was higher in summer- (38.2+/-0.3 degrees C) than winter-acclimatized (37.1+/-0.5 degrees C) birds. Below the TNZ, resting metabolic rate (RMR) increased linearly with decreasing T(a), and RMR and C were higher for summer- than winter-acclimatized birds. The mean mass-specific BMR of owlet-nightjars (1.27 mL O(2) g(-1) h(-1)) was close to the allometrically predicted value for a 45 g Caprimulgiformes, but well below that predicted for birds overall. These results suggest that owlet-nightjars increase plumage insulation to cope with low winter T(a), which is reflected in the seasonal difference in RMR and C below the TNZ, rather than adjusting BMR.