An investigation of birds' legs as thermal windows using infrared thermography

Bird bills and legs are well established in their importance for thermoregulation. However, bills have been the focus of most contemporary research quantifying the use of bird appendages as thermal windows (i.e areas of controlled heat loss). Using non-invasive infrared thermography, I aimed to investigate the use of birds? legs as thermal windows, how these compare to the use of bills, and whether different species varied in their regulation of heat loss through their legs. I collected 430 thermal radiometric images and videos over a range of climatic conditions for 11 species of wild birds around Melbourne, Australia. With these thermal images and videos, I was able to gather the mean surface temperature of birds? tarsi (their exposed lower legs), feet, bills, and feathered body and analysed the relationship of these surface temperatures to the corresponding apparent temperature of the environment. I found that in cold conditions, birds were able to lower the surface temperature of their tarsi and feet below the temperature of their non-vascularised feathers to retain heat in their core. As conditions became warmer, there was an increase in the surface temperature of their tarsi and feet such that they were warmer than the feathered body, indicating the capacity to expel excess heat. By contrast, the surface of bird bills was consistently approximately 2?C above that of the non-vascularised feathered surface causing heat loss at both lower temperatures as well as warm temperatures, suggesting that birds have less capacity to regulate heat loss through bills compared to their legs. Magpie-larks showed significant difference to Australasian swamphens in the rate of change in surface temperature as apparent temperature increase while silver gulls and Australian magpies did not, indicating a greater capacity to regulate heat loss through the legs in magpie-larks. The vascular configuration to facilitate counter-current heat exchange in bird legs but not bills may explain why they are more efficient thermal windows, and why bird bill morphology shows a greater relationship with temperature (associated with Allen?s rule) than do bird legs.