Air temperature limits aerobic capacity and scope in active birds

Thermal tolerance limits are an important determinant of species vulnerability to climate change. However, most of our knowledge of thermal tolerance pertains to resting animals, which could yield unrealistic predictions about how air temperature will affect animals when they are active. To gain a better understanding of thermoregulatory contstraints in active birds, we induced zebra finches in captivity to exercise continuously in air temperatures ranging from cool to warm (12- 36 C) while we simultaneously measured their metabolic rate, body temperature, and evaporative water loss. During activity, birds overheated in air temperatures well below their resting thermal tolerance limit, and aerobic scope was reduced by 22% at the warmest temperature. Although zebra finches showed a 8-fold increase in evaporative water loss during exercise in the warmest temperature, this was not sufficiently high to compensate for the substantial increase in heat produced during activity. This could explain why birds became hyperthermic and showed reductions in metabolic rate in the higher air temperatures. Thus, our results provide a mechanistic explanation for how heat could limit aerobic capacity and scope in active birds. These findings support growing evidence that increased air temperature can lead to sub-lethal fitness costs in free-living birds.