Have you ever marveled at the extraordinary journeys that some birds go on when they’re feeding or migrating? Have you ever wondered how birds are able to fly through the forest so quickly, yet not crash into things? Have you ever wondered how it is that many birds can remain on the ground as a car is approaching and then quickly escape at the very last second? You haven’t? Oh well, neither had I until a few years ago. Now, though, I think about it almost every time I see a bird. For I now know that underpinning these observations is something quite special about birds that explains so much about why birds are the way they are. 

Birds are hot(1). In fact, they’re the hottest vertebrates (2) around. You and I maintain a more or less constant body temperature of around 37°C. That’s fairly typical for a mammal. Birds also maintain a relatively constant internal body temperature—except theirs is likely to be in the range of 39– 43°C, i.e. quite a bit higher than your average mammal. Now you don’t need to know much chemistry—or baking for that matter—to know that when you heat things up, chemical processes speed up. This is what happens in birds. Stuff happens more quickly. For instance, reaction times are faster and muscles can work harder. Fast reaction times explain how birds can dodge a moving car and how a bird can fly around the forest at speed without crashing. The long journeys some birds undertake are made possible by having muscles that can work hard (as is being able to fly around the forest at speed in the first place). Birds simply couldn’t do these things if the chemistry in their bodies wasn’t in overdrive, and the chemistry in their bodies wouldn’t be in overdrive if they didn’t have high body temperatures. 

Birds are really approaching the limit of what is possible for a vertebrate. The absolute internal temperature limit for vertebrates seems to be about 46°C. Anything above that can cause brain damage that’s fatal. Despite this, there are even birds that maintain a body temperature of around 45°C!(3) 

Having a high body temperature is hard work. It’s unsurprising then that a bird’s body is fine-tuned for maximum performance. More energy is needed, so birds need to eat more and/or use their energy more efficiently. Oxygen needs to be pumped round the body more efficiently. Waste products need to be removed more rapidly. These are just some of the obvious fine tunings a bird has. If vertebrates are likened to cars, then birds are the Formula 1 racing cars. A tweak here, a redesign there—all for optimum efficiency. 

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It would be possible to write a book on the many ways a bird is fine-tuned for high performance. I’m just going to take one example to illustrate the point: getting oxygen into the body. Bird lungs are quite different from our lungs, and they breathe quite differently too. Everything about the respiratory system of birds is optimised to get as much oxygen into the blood as quickly as possible (and carbon dioxide out at the same time). 

When humans breathe in, air goes into the lungs and ends up in numerous little sacs called alveoli (see diagram above). Once there, oxygen passes across the lining of the alveoli to enter the blood. At the same time carbon dioxide passes the other way. When we breathe out the carbon dioxide is expelled.

That’s all very well and good. But there’s a lot of time in a breath when there’s not much oxygen going into the blood. When we breathe out, we’re just getting rid of carbon dioxide sitting in the lungs. Oxygen doesn’t return until the next breath. What if lungs could be designed so that there is always oxygen going into the lungs? That would be so much more efficient. 

And that’s precisely what birds do. Birds have a one way system in the their lungs. In order to facilitate this, they have something else as well: air sacs for storage (see diagram). There are sacs towards the head of the bird and sacs towards the rear. Some of these sacs are even in bones. (This has the added advantage of making the bird lighter than it would be if the bones were solid, thus helping with flight, but that’s another story.) The air sacs are connected to the lungs. The lungs have numerous tubes called parabronchi (see diagram). Like the alveoli in mammals, oxygen enters the blood across the lining of the parabronchi and carbon dioxide leaves. However, unlike alveoli in mammals, air does not go in and out of the parabronchi, but flows in one direction only. Not only that, fresh air is arriving into the parabronchi whether the bird is breathing in or out.

How does that work you may ask? This is where the air sacs come in. They act like bellows to the parabronchi. When a bird takes a breath in, air enters the rear air sacs and the entrance to the lungs (see diagram above). When the bird breathes out, these air sacs pump air into the parabronchi. On the next breath in, that air will then go into the air sacs near the head (and the exit to the lungs). On the next breath out, that air will be exhaled. In this way, fresh air is always entering the parabronchi. Really efficient, isn’t it? There are other efficiency tweaks too. For instance, the lining of the parabronchi is much thinner than the lining of the alveoli in mammals. This allows oxygen and carbon dioxide to pass more quickly to and from the blood respectively. Birds are able to have this thinner lining because their lungs don’t inflate and deflate like mammal lungs do. The lining of the alveoli in mammal lungs needs to be able to cope with being stretched when air enters. The lining of the parabronchi in birds doesn’t need to stretch so can get away with being thinner. 

The list of fine tunings birds have goes on and on. Formula 1 racing cars indeed! I will finish though with a little coda on our beloved kiwi. You may have noticed I mentioned the upper limit for the internal body temperature of a bird, but didn’t say anything about the lower limit. It turns out that the kiwi is more or less at this lower limit. It has one of the lowest internal body temperatures of any bird, if not the lowest. The body temperature of a kiwi is around 37–38°C (with one study measuring it 36–37°C). (4) This is about the same as us. Not only this, kiwi don’t have air sacs in their bones. Their bones are filled with marrow like ours. These are yet more ways in which kiwi are more like a mammal than a bird. The DOC team who monitor kiwi could well have observations on the dodging speed of a kiwi compared to other non- or reluctant-flying birds on the motu. 

(1)Although visitors to Tiritiri Matangi may argue that birds are really cool. (Sorry, I couldn’t resist the dad joke!)

(2) Loosely speaking, vertebrates are those creatures most people would call animals. They comprise mammals, reptiles, amphibians, birds, and the various forms of fish (including sharks and rays). 

(3) The highest known body temperature appears to be in the Somber Hummingbird from Brazil (Eupetomena cirrochloris). Having said that, a recent study has shown that the red-billed quelea from Africa (Quelea quelea) can tolerate a temporary body temperature around 48–49°C, although the brain is kept cooler than this. See https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7403380/. 

(4)See https://sora.unm.edu/sites/default/files/journals/auk/v113n03/p0687-p0692.pdf for a discussion of the body temperatures of kiwi. The lower temperature comes from an earlier study (referred to in the linked article) and was a daytime temperature. Daytime is when kiwi normally sleep, and so one would naturally expect a lower temperature then. 

References:

• Gill, F.B. & Prum, R.O. 2019. Ornithology (4th ed.). W.H. Freeman and Company, New York. (Chapter 6 in particular)
• Pough, F.H, Bemis, W.E, McGuire, B. & Janis, C.M. 2023. Vertebrate Life (11th ed.). Sinauer Associates, New York. (pp. 296–297) 

Diagrams from Wikipedia (shared through Creative Common licensing):

• https://commons.wikimedia.org/wiki/File:Human_Lungs.gif
• https://commons.wikimedia.org/wiki/File:Cranial_sinus_and_postcranial_air_sac_systems_in_birds .jpg
• https://commons.wikimedia.org/wiki/File:Bird%27s_respiratory_system.jpg
• https://commons.wikimedia.org/wiki/File:Bird_Respiration_-_Air_circulation_%28FR%29.svg#file (edited to replace French labels with English ones)