Images from a thermal camera showing areas of the dove's body losing heat after resting, after being in wind, after being under a heat lamp and post-flight.
During the winter, it can be comforting to snuggle beneath a duvet, stuffed with warm down and feathers, and if the heat gets too much, just kick it off. But birds aren't so lucky. Their feather coats are in place 24/7; there's no option to shrug them away when they get too warm. Fortunately, birds are not entirely enveloped in feathers. Their feet and lower legs are bare, the skin around their eyes tends to be feather free and their bony beaks are packed with blood vessels, to radiate heat. In addition, birds raise their wings slightly away from their bodies to keep cool when overly warm. But how do birds combine these various strategies to remain cool on balmy days, yet maintain warmth when buffeted by a chilly breeze? Kristen Crandell (Bangor University, UK) and Bret Tobalske (University of Montana, USA) teamed up with thermal imaging expert Donald Powers (George Fox University, USA) to find out how doves (Streptopelia decaocto) manage their body temperature on warm and windy days and when flying.
First, the team gently inserted a tiny electronic thermometer into each dove's abdomen, to monitor the birds’ core body temperatures. A few days later, they attached a second electronic thermometer to the bird's breast, to record the skin temperature beneath the feathers. They then rigged up a thermal imaging camera to record the birds’ external temperatures, from the feather-free lower legs to the face and the surface of the plumage. Once all three temperature trackers were in place, the trio allowed the birds to rest on a perch for 10 min before recording their skin and body temperatures and thermally imaging their bodies to identify regions that were glowing with warmth and how large they were. Then, they shone a heat lamp on each dove for 10 min, to simulate a sunny day, before blowing a fan on the birds to mimic a feather-ruffling breeze. And finally, they encouraged the birds to fly repeatedly between two perches, monitoring the birds’ body, skin and external temperatures after each event.
Impressively, the doves maintained a reasonably stable core body temperature, around 42.5°C, regardless of the conditions – rising a little after flying and dipping slightly after being in a breeze. However, the skin temperature of the doves that had been perching in a light wind was only 39.3°C, in contrast to the flying, sunning and resting birds, whose skin temperatures were around 41°C. ‘Skin-level temperatures do not necessarily reflect patterns in the core body temperature’, says Crandell. But how were the birds able to maintain such a constant core body temperature when the conditions that they were experiencing impacted their skin temperature so dramatically?
The team analysed the thermal images and noticed that the areas of skin around the eye, wings and lower legs were colder when the birds were cooled by a breeze, and the cool area encircling the eye was smaller, allowing the birds to retain warmth in their bodies when chilled by wind. In contrast, when the birds emerged from beneath the heat lamp, those body regions remained at the same temperatures that they had been at when resting in still air. However, the areas of skin losing heat around the eye and wing were larger to keep their core temperature down.
Instead of altering their overall skin temperature to maintain a stable core body temperature, the doves adjusted the area and temperature of key regions of skin that lose heat, like mini radiators. In addition, the birds’ head temperatures were generally 2°C cooler than their feet, likely to protect their brains from overheating, no matter what they are up to.
