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 infra-red 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. In
contrast, bill surface temperature remains ~2˚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.