Abstract For birds that migrate long distances, maximizing the rate of refueling at stopovers is advantageous, but ambient conditions may adversely influence this vital process. We simulated a 3-day migratory stopover for garden warblers ( Sylvia borin) and compared body temperatures ( T b) and rates of refueling under conditions of a heat wave ( T a = 40 °C by day, and 15 °C at night) with those under more moderate conditions ( T a = 27 °C by day, and 15 °C at night). We measured T b with implanted thermo-sensitive radio transmitters. Birds had significantly lower rates of body mass gain on the first day of stopover (repeated measures mixed model ANOVA, p = 0.002) affecting body mass during the entire stopover ( p = 0.034) and higher maximum T b during the day when exposed to high T a than when exposed to moderate T a ( p = 0.002). In addition, the birds exposed to high T a by day had significantly lower minimum T b at night than those exposed to moderate daytime T a ( p = 0.048), even though T a at night was the same for both groups. We interpret this lower nighttime T b to be a means of saving energy to compensate for elevated daytime thermoregulatory requirements, while higher T b by day may reduce protein turnover. All effects on T b were significantly more pronounced during the first day of stopover than on days two and three, which may be linked to the rate of renewal of digestive function during stopover. Our results suggest that environmental factors, such as high T a, constrain migratory body mass gain. Extreme high T a and heat waves are predicted to increase due to global climate change, and thus are likely to pose increasing constraints on regaining body mass during stopover and therefore migratory performance in migratory birds.