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.<p>Funding provided by: Australian Research Council<br>Crossref Funder Registry ID: http://dx.doi.org/10.13039/501100000923<br>Award Number: DP190101244</p><p>Funding provided by: Natural Sciences and Engineering Research Council of Canada<br>Crossref Funder Registry ID: http://dx.doi.org/10.13039/501100000038<br>Award Number: RGPIN-2020-05089</p><p>Dataset includes surface temperature estimates of bird bills, legs, eyes regions and plumage taken from infra-red thermographs of wild birds, as well as microclimate data collected in the field. Please see our paper for full details. </p&gt