Avian Metabolism and Thermoregulation

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Having just recently moved to the Keys from Pennsylvania, I’ve spent the past few weeks adjusting to this unusually warm “winter.” After sweating buckets from just a few hours at the beach (in December!), I began to ponder how animals, such as birds, live outside all day every day and manage to maintain a stable body temperature. Knowing how often injured and ill birds come to the Wild Bird Center with low temperatures and how imperative it is to prioritize raising body temperature, this topic became even more intriguing. After a little bit of research, I learned a lot about how amazing birds’ bodies are!

At the root of it all is a bird’s metabolism, all of the chemical processes occurring in its body to maintain life (ii). These chemical reactions do all sorts of things – build and repair tissues, process information, produce eggs, and any other activity in the body. All of these processes require energy, which comes from the food they eat. Birds have high metabolic rates, meaning they use a lot of energy at high rates in order to survive (i). That being said, birds must eat a lot (relative to their body size) to thrive in the wild. So the phrase “eat like a bird” should actually have a very different meaning!

One of the main reasons birds use so much energy is because they are endothermic, which is a fancy word for warm-blooded (iii). Birds maintain their body temperature metabolically regardless of the temperature of their environment. Because of this, birds can live in a wide range of habitats. This is part of the reason we find birds everywhere from the Artic to the tropics. However, maintaining a stable body temperature metabolically requires a LOT of energy. Up to 90% of a bird’s metabolism, or energy, can be devoted to maintaining its body temperature (iv). Healthy birds naturally have rather high body temperatures, typically ranging from 38-42⁰C, whereas most mammals range from 36-39⁰C (iv). Birds also incubate their eggs outside of their body, which requires them to produce more heat while nesting.
As a general rule, metabolic rate and mass are inversely related (i). Pound for pound, smaller birds expend more energy than larger birds. To minimize this energy cost, birds of all sizes have physical adaptations to help them gain and lose heat to maintain their body temperature. For example, although birds do not have sweat glands, they can lose heat through their respiratory system and exposed skin (iii). Much like in dogs, birds can effectively “pant,” allowing air to pass over moisture in their respiratory system to removes heat through evaporation.

Moreover, birds’ veins and arteries in their legs are very close together, allowing for heat exchange between blood flowing in each direction (i). In this way, if blood from a bird’s feet is cold, then it’ll gain heat as it passes next to blood coming directly from the heart. In extreme cold or food scarcity, on the other hand, birds can enter a state of torpor, meaning that they lower their body temperature and slow their heart rate, respiration, and metabolic rate to conserve energy (ii).


Birds have also developed behavioral traits that help them maintain their temperature. Puffing up their feathers, for instance, allows them to create air pockets that trap body heat and keep them warm, while compressing their feathers against their body releases those pockets (iii). They also will pull up one leg or lower their bodies on top of their legs, tucking their beaks into their feathers, to minimize the amount of their bodies that are in direct contact with the cold air (iii). Even simply changing their posture or orientation to the sun can impact their body temperature, so birds will seek out the perfect sunny or shady spot (iii). Some birds, such as penguins and some species of songbirds, exhibit huddling behavior in cold weather, particularly while sleeping, to share body heat and minimize the energy spent on keeping warm (iii).


Canada geese curling up to keep warm.


Tree swallows huddling together for warmth.

While this overview only scratches the surface of the intricacies of birds’ bodies and their temperature regulation, it shows how awesome birds’ bodies are and how well-adapted they are to their surroundings. It also shows how sensitive their temperature balance is, and just like in humans, illness and injury can impact their temperature. As such, one of the first things we do for every bird that comes into our hospital is take their temperature, and we also periodically check it for the duration of their rehabilitation. This helps us save birds’ lives and return healthy, rehabilitated birds to their natural habitat!

Jessica Coulter, Hospital Intern, 2016

(i) Ritchison, Gary. Eastern Kentucky University.
(ii) Stanford University
(iii) Stanford University
(iv) Ornithology.com

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