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 (Tb) and rates of refueling under conditions of a heat wave (Ta = 40 °C by day, and 15 °C at night) with those under more moderate conditions (Ta = 27 °C by day, and 15 °C at night). We measured Tb 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 Tb during the day when exposed to high Ta than when exposed to moderate Ta (p = 0.002). In addition, the birds exposed to high Ta by day had significantly lower minimum Tb at night than those exposed to moderate daytime Ta (p = 0.048), even though Ta at night was the same for both groups. We interpret this lower nighttime Tb to be a means of saving energy to compensate for elevated daytime thermoregulatory requirements, while higher Tb by day may reduce protein turnover. All effects on Tb 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 Ta, constrain migratory body mass gain. Extreme high Ta 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.