Reduced body mass gain in small passerines during migratory stopover under simulated heat wave conditions

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.