Birds are better at regulating heat loss through their legs than their bills: implications for body shape evolution in response to climate

Endotherms use their appendages-such as legs, tails, ears and bills-for thermoregulation by controlling blood flow to near-surface blood vessels, conserving heat when it is cold, and dissipating heat in hot conditions. Larger appendages allow greater heat dissipation, and appendage sizes vary latitudinally according to Allen's rule. However, little is known about the relative importance of different appendages for thermoregulation. We investigate physiological control of heat loss via bird bills and legs using infrared thermography of wild birds. Our results demonstrate that birds are less able to regulate heat loss via their bills than their legs. In cold conditions, birds lower their leg surface temperature to below that of their plumage surface, retaining heat at their core. In warm conditions, birds increase their leg surface temperature to above that of their plumage surface, expelling heat. By contrast, bill surface temperature remains approximately 2 degrees C warmer than the plumage surface, indicating consistent heat loss under almost all conditions. Poorer physiological control of heat loss via bird bills likely entails stronger selection for shorter bills in cold climates. This could explain why bird bills show stronger latitudinal size clines than bird legs, with implications for predicting shape-shifting responses to climate change.

Automated summary

This study used infrared thermography to investigate the physiological control of heat loss via bird bills and legs. The results showed that birds are less able to regulate heat loss via their bills compared to their legs. In cold conditions, birds lower their leg surface temperature to retain heat, while in warm conditions, they increase their leg surface temperature to expel heat. By contrast, bill surface temperature remains consistently higher than the plumage surface temperature, indicating consistent heat loss. This poor physiological control of heat loss via bird bills likely leads to stronger selection for shorter bills in cold climates and has implications for predicting shape-shifting responses to climate change.