Scott Taylor: It's the highest peak in the Americas, nearly 23,000 feet of Argentinian Andes going straight up. And I want to be clear that I was not there to climb it. I was there to look at it from the ground with a camera. I've been down in South America sampling seabirds off the coast of Chile, as you may remember from our episode on boobies. And on the way out, a colleague and I drove up to see the mountain. We parked around 11 1/2 thousand feet, which at the time was the highest I'd ever been. That number doesn't really faze me. Now, the field station I direct runs from 9500 feet up to 12 1/2 thousand. So that's basically my office, but back then it was new. I was excited and so I did the dumbest thing a sea level body can do with that elevation. I tried to run. I only made it a few seconds before I was bent over, hands on my knees, lungs on fire. Up there. The air is still 21% oxygen, same as the beach. There's just less air pressure so your body can't grab it as well. You feel it immediately and that's at 11,000 feet. People feel it at a fraction of that. Denver doesn't get called the Mile High City as a bit. Tourists fly in and feel one drink like it was 3, and touring comedians often keep a little can of oxygen on stage for hits between jokes. A single mile up is enough to throw a sea level body way off. I was sitting at more than double that, very much unable to catch my breath, and the whole time I'm doubled over feeling my heart race. There are birds in the air above me, not struggling, not gasping, just flying, going about their afternoon in the exact air that had nearly taken me out. And flying is hard. It's about the most oxygen expensive thing a body can do, far more than my sad little jog. To them, it was just a Tuesday.

Scott Taylor: Which brings us to the topic of today's episode. OK, but what's it like as a bird at the top of the world? To dig into this, we'll talk with Dr. Chris Witt, professor at the University of New Mexico and Curator of Birds at the Museum of Southwestern Biology, who has spent his career figuring out how birds make a living in the thinnest air on Earth. After the break, Chris walks us through what actually changes inside a bird that lives at the top of the world, and how strangely predictable that change turns out to be. We get into why a single mountainside can stack dozens of species right on top of each other. And we get to the record holders, the birds pulling off things up high that sound like they should not be possible. As a heads up, Chris doesn't just tell us about these awesome birds, he shows us, so you might want to watch this one if you're just listening. Stay tuned.

Scott Taylor: Well, welcome back to the podcast, everyone. I'm really excited to have Chris with us today. Thanks for joining us, Chris.

Chris Witt: I'm a big fan of this podcast, so this is a great honor to be here with you, Scott.

Scott Taylor: Awesome. Well I'm glad to hear you're a fan. It's been really fun to put together the space behind you. Chris is really cool looking. Can you tell us a bit about where you are right now?

Chris Witt: I'm, I'm joining you from the Museum of Southwestern Biology, which is our natural history collections at the University of New Mexico. So the room I'm in has about 400,000 birds and mammals that are preserved. They're dry, they're skins and skeletons. If you go to the the floor above us, we've got a cryopreservation facility where we've got frozen tissues from most of the animals that are in this room and a lot more. And the purpose of this is to facilitate research and education on animals.

Scott Taylor: Amazing. So most people think of the challenge that altitude poses to humans, right? So altitude sickness is something we might experience if we go up in elevation too quickly. Often times athletes train at high elevation in order to be more competitive at low elevation. Or people might think about the fact that they want to pack supplemental oxygen when they come to Denver. So what does it actually mean to be a bird living its entire life above like 11-12 thousand feet?

Chris Witt: Great question. Yeah, the human analog is perfect. Like we are experienced. Changing elevations is a perfect analogy for what birds experience when they change elevations, with the caveat that the mammalian respiratory system is fundamentally different in its design from the avian one. But the same basic principles apply. We're trying to breathe in air, take up oxygen at the lungs, and then distribute that oxygen through our bodies with our blood to the cells that are doing the hard metabolic work.

Scott Taylor: Yeah. I thought maybe before we get into like the actual adaptations to elevations, we could talk a little bit about what's driving that kind of diversity and how this elevation piece plays into the generation of some of this diversity in places like the Andes.

Chris Witt: Yeah, I would say there's no question that I was inspired to study birds and elevation because of going to the Andes, because it's so dramatic how the bird community changes from elevation to elevation. If you go up the mountain or down the mountain, you see complete turnover and that's really exciting. You have a elevation defines distributions in those mountains. The diversity from place to place is very high. It's it's not that way in the temperate zone. So when we should start there? Temperate zone mountains that are seasonal are fundamentally different from tropical zone mountains that are not seasonal and where the birds are staying at more or less in the same place all year round. That has a huge impact on the evolution of the whole system and the diversity of the system and the extent to which elevation itself is important for the biology of these birds in the tropics. When they're in the same place all year round, they specialize on an elevation and it has an impact on their elevation distribution and also on their their their evolution and diversification.

Scott Taylor: Yeah, it's interesting to think about that, that that difference between temperate and tropical mountains. I mean, I've spent time in both places as well. And here in the Rockies, you know, you see the American Robin all the way from basically sea level on one part of the country all the way up to breeding on, on the tundra. And so they're not as tied to these elevation bands, while they're really not tied to elevation bands in any way compared to these tropical birds. A lot of your research, you know in the Andes is focused on hummingbirds and and hemoglobin, which we should definitely talk about hemoglobin, what it is, what it does and the fact that this hummingbird hemoglobin is evolving to match elevations.

Chris Witt: The Center for bird hummingbird species diversity is in the Andes in middle elevations, and when you go there, you see that hummingbirds are very specific to their elevations. And in fact, if you look across all Andean hummingbirds, the average elevation breadth of the range is only 1100 meters. So you know, a little over 3000 feet from top to bottom. That means the species doesn't move below that point or above that point. So it's very strange, very peculiar that they're so restricted in elevation, even though individuals of this species would have no trouble flying from the bottom to the top of the mountain. They just don't do it. The species is constrained within those bounds. So this set up a clear question, why? And the first thing that our research group looked at with that was the hemoglobin protein, because that protein is so essential to oxygen uptake and distribution. It's the most abundant protein in our bodies and it's responsible for grabbing those oxygen molecules at the blood gas barrier in the lungs and then distribute it through the body to the cells that are actually doing the metabolic work, and then releasing the oxygen to those cells to support aerobic metabolism.

Scott Taylor: Am I getting this right? Lower pressure is higher up. It's harder to grab the oxygen. That's why like if a human who is adapted or lives at sea level comes to Colorado and goes directly to Rocky Mountain National Park, which much of which is above 10,000 feet, it's harder for them to breathe. It's harder for them for their hemoglobin basically to grab onto that oxygen. That's that's the thing that they notice, right?

Chris Witt: Exactly. So you can measure this by with a, with a pulse oximeter like people have that on their fitness watches. Now you can measure blood oxygen saturation exactly. So this has become a very familiar measurement and you can look at your blood oxygen saturation if you're at sea level, it's always 99% or 100%. It's because it's very easy for the for human hemoglobin to load oxygen and see when you're breathing in sea level air. As you go up though, that blood oxygen saturation drops below 100%. Even if you're acclimatized and ready, you know your body's ready to to manage that. So right now you and I are each about a mile high in Albuquerque and and Boulder, respectively. So our oxygen saturation is a little below 100%. It's probably in the mid or high 90s and it will keep going down as you go up in elevation and it actually eventually will will start to drop off precipitously. It's nonlinear relationship that has a lot of consequences for for function. And you know, when highly trained Everest climbers are nearing the the peak of Everest, their blood oxygen saturation is typically below 50%. So it's very, very low at those elevations, but the acclimatization that they've undergone allows them to cope with that and still continue to produce a lot of metabolic work to be able to climb.

Scott Taylor: But it is crazy, yeah. Below 50% is insane to think about like a human body still being able to function at that high of an elevation. And I guess it makes sense why it's such a risky endeavor for those for those Everest climbers. So then from the bird perspective, one of the things that happens is that their their hemoglobin evolves to be better right at grabbing the oxygen even at lower pressures and and subsequently lower concentrations. Well, I guess same concentration, lower pressure.

Chris Witt: Exactly. So the hemoglobin molecule itself is incredibly complex. It's not that big. It's 4 subunits, each are about 140 amino acids long. And then they fold together in this unique, amazing way that allows them to grab oxygen, transport it, and release it for us. It's very sensitive to that oxygen pressure, but very fine changes to the amino acid sequence in those four subunits can affect how readily it grabs oxygen so that it might saturate at a lower oxygen pressure. So adaptation happens by DNA changes, mutations that are favored by natural selection. They result in amino acid substitutions or wholesale switches in a particular population that affect that affinity. So you can imagine if a population is evolving at a lower oxygen pressure, it's breathing in fewer oxygen molecules with each breath. Over time, you're going to get natural selection for a hemoglobin protein that more readily binds to oxygen, so that's able to load with oxygen better at the blood gas barrier. And sure enough, when we looked at these Andean hummingbirds across elevations, we found that they very predictably had these amino acid substitutions, which are evolutionary changes favored by natural selection that relate to the elevation at which they live. And we weren't surprised to find those adaptations, but the degree of predictability was shocking. Every time there was a movement up, you'd get a particular substitution. And then if it, if you go way up, you get another substitution. And those things would happen repeatedly through the hummingbird phylogeny. And then one of the really cool things is that when you get a reversal, a species moving down, you get a reversal of the hemoglobin. So it's predictable in both directions. And it's happened dozens of times across the hummingbird tree. So what that indicates to us is that there's a fine tuning going on of the protein structure that's responding to shifts in the elevation at which these species live.

Scott Taylor: It's amazing that there's a reversal to how like what's the evolutionary timescale of the reversals compared to the like the hemoglobin evolving to be better at capturing oxygen versus kind of reverting to being not as good at lower elevations?

Chris Witt: That's a great question. I mean, we don't know like how long it takes for these outages to happen. We just know they happen really predictably. And we also know that when we look at species that span elevations or maybe where we have populations that are at different elevations in different parts of the Andes, we see polymorphisms. That is, we see both a high altitude allele or gene variants and a low altitude gene variant in the same population and they, you know, when they sort out across elevation. So that suggests that this happens pretty fast. These adaptations happen pretty quickly. They certainly are very highly predictable, but there still might be a lag between when a a shift in elevation happens and when the adaptation catches up to that shift. And one of the reasons that that might be the case is that the hemoglobin protein is so complex that any change in the hemoglobin protein anywhere is likely to interact with any other change. And so over time, any bird lineage will accumulate changes that happen by chance. The protein's extremely, extremely constrained, but it can accumulate amino acid changes that don't have a lot of effect on fitness, but they may have an effect on which other changes are subsequently possible because they're going to interact with whatever changes have accrued. So that means that the adaptive paths in terms of like change this amino acid, then that one, then that one that are available to any given species are very limited. And we see hummingbirds take one predictable path and reverse it when they shift elevation. But if we look out to other groups of birds, warblers, flycatchers, ovenbirds, pigeons and doves, etc., we see different paths in all those kind of ages.

Scott Taylor: Cool.

Chris Witt: So the predictability that we see in hummingbirds, it breaks down if you look at more diverged hemoglobin proteins that are found in other birds.

Scott Taylor: How does the various ways that hemoglobin evolves in birds with respect to high elevation adaptation compared to humans? Because there are some populations of humans that are evolved to live at high elevations, right?

Chris Witt: Yes, this is this is gets to some very complicated physiology and biochemistry, but I'll try and kind of summarize it at a high level level there. I understand it at humans have so many known hemoglobin variants like there's a catalog of four or 5000 human hemoglobin variants that are known and they cause all kinds of pathologies. You know, you can think of one. Everyone knows about sickle cell anemia, which is a hemoglobin variant. But what's really interesting is that none of them, none of the thousands, are altitude adaptations. So lots of variants, but none of them are adapted for altitude and have been so seized on by natural selection in human populations. So unlike birds, humans don't have hemoglobin structural adaptations.

Scott Taylor: OK.

Chris Witt: But there aren't really any substantial populations that are permanently living at those elevations. That makes sense because. Of that we don't have hemoglobin adaptations in our gene pool, they just don't exist. And we, we assume it's because, you know, the fundamentally different design of our respiratory system wouldn't make those particular changes advantageous until we reached our extreme level. There's a, a paper from the 60s by Vance Tucker where he, he took some house sparrows and he took some house mice and he put them in a, a low pressure chamber to simulate altitude. And he brought them up above 6000 meters. So this is sort of somewhere between Everest base camp and the Everest peak, right? He brought them straight up, and the mice just went comatose, and they just went on this trajectory towards death. And house sparrows were absolutely fine just hopping around the cage. And so that's, you know, a direct consequence of the sort of differential ability of birds versus mammals to adjust and adjust quickly to differences in altitude.

Scott Taylor: Yeah. I mean, it's amazing to think about, you know, when you're flying in a jet going somewhere on the planet that there are certain birds that just fly at that elevation like that altitude. Like bar-headed geese are actually flying over the Himalayas. They're not just going over, they're flying powered flight at that elevation, at that low partial pressure of oxygen and they can still just do it. And there are many other bird examples, including in your own work. Like you found hummingbirds can keep flying even when oxygen drops to. Is it 6%? So like equivalent to 43,000 feet?

Chris Witt: That's about right, yeah. So we know like the classic example that people cite as this jet collision with a vulture.

Scott Taylor: Oh right, the Ruppell's griffon vulture.

Chris Witt: Exactly. The Ruppell's griffon vulture over Ivory Coast 1973, a jet flying at 37,000 feet. So that's over 11,000 meters. That's well above Everest. You know, it's Everest plus another mountain on top of that. And you know, it's, it's a cool anecdote because it's just absolutely unquestionable, you know, because the remains of a Ruppell's griffon vulture were on the airplane and the pilot knew what elevation he was at. And so, you know, it's, it's still an incredible example of the ability of birds to go high with few limits. Now that vulture though, it was gliding, right? And we know what these vultures do. They ride these warm air thermals to very, very high elevations or altitudes. But they they're not doing a lot of work. You know, they're just moving their primaries a little bit, you know, to adjust to circle, to find these air columns. And then once they get that kind of altitude, they can go anywhere. You know, they can go across the continent of Africa easily in a day, the ones they've gotten up there. So that's cool.

Chris Witt: But hummingbirds are really different. They don't soar, they don't rest. You know, the biggest hummingbird has a 18 wing beats per second. The smallest ones get up to 70 wing beats per second. So, so you know, a typical mid size 1 is 50 wing beats per second. We've tested the ability of hummingbirds to withstand low, low oxygen. We do it by reducing the fraction of oxygen in a in a flight box, but it's the same effect on the oxygen uptake at the lungs. And when hummingbirds are hovering, they have to raise their metabolic rate about 10 times above resting, tenfold above resting. So they're consuming a lot of oxygen. And what we found is they're incredibly robust to reductions in the oxygen fraction that simulate high altitude so much that any hummingbird among the many that we've tested can continue to hover at about the elevation of Mount Everest. So this is 8800 meters, almost 30,000 feet. And they can continue to hover if you slowly, gradually take them to that elevation, even if they're not high altitude adapted. Then if we look at the very highest hummingbirds, the ones that are the the champions of high altitude, those are the ones that can go above 40,000 feet. They have just truly exceptional. Ability to resist low oxygen just means that it's a testament to how efficient they are at taking up oxygen, even when oxygen is very scarce. Those elevations, if we, you or I, went up there now, we would drop into a coma immediately and our lives would be, you know, really threatened if we didn't get brought down very quickly.

Scott Taylor: So what? Species are those. What are those? Those amazingly high elevation adapted hummingbirds.

Chris Witt: Yeah, I have some of them behind me. Actually brought some of them out.

Scott Taylor: Nice.

Chris Witt: So this is a black-breasted hillstar. It's got this beautiful green throat, black breast, tan sides.

Scott Taylor: Beautiful.

Chris Witt: Yeah. The hummingbirds in this genus are all incredibly robust. The genus is Oreotrochilus. They're called the hillstars. There's a handful of species, and all of them can easily go up to about 5000 meters. It's very hard to find hummingbird habitat above that, but undoubtedly they could go above that if they if they had flowers to make it worthwhile.

Scott Taylor: Amazing. When I saw a sword-billed hummingbird for the first time I was just like what? What the heck? Do you have one?

Chris Witt: Oh man, of course.

Scott Taylor: I don't know. For me, seeing them feed on the flowers they've evolved to feed on at elevation, like look at that bill. It's just insane.

Chris Witt: It's incredible.

Scott Taylor: Oh my God.

Chris Witt: So I've got some some species here that we call elevation replacements. These are species in the same clade. These three are in the same genus. Here's one that's in the lowland Amazon. It's called a Gould's jewelfront. And then here's the.

Scott Taylor: Jewelfront a little bit, yeah.

Chris Witt: And then here's a a relative that's kind of in the foothills we call the subtropical zone be about 1000 to 2000 meters and you go up another 1000 meters, you get this one, the fawn-breasted brilliant. And then this one's a little more distantly related, but if you go up above 2500 and 3000 meters, you have the swordbills.

Scott Taylor: Yeah, they're. There with the exclusive access to certain flowers that have coevolved with them over the millions of years. I don't know the name of the flower, but there's this one orange kind of bell hanging flower that I saw them feeding out of when I was in Ecuador. One thing that I guess we've talked to a lot of our guests about, you know, the world is changing pretty rapidly in the context of carbon dioxide concentrations in the atmosphere and the impact they're having on overall temperature. And as temperatures rise, a lot of these high altitude species that you're working on of hummingbirds, they don't have anywhere else to go. And so I know that, like, climate change isn't necessarily a huge focus of your work, but what does it like? What does it look like as a threat for these birds, these high elevation adapted hummingbirds that really, you know, don't have anywhere to go?

Chris Witt: Yeah, climate change is affecting all of us. It's certainly going to affect all birds. But I, I'm glad you raised this issue because it's really important that we think about all the different things that vary with elevation. We talk about high elevation or high altitude adaptation and and we mentioned that pressure is going down by about 10% every 1000 meters, but temperature is also going down by about 6° C every 1000 meters on a dry in a dry environment and a little bit less than that in a humid environment. But these changes in temperature are really fundamental and people have a great understanding of how temperature effects life, both diversity and also traits across temperature gradients around the world. So we have a good understanding of what temperature does, but we don't have as good of an understanding of what pressure does. And of course, cross elevation of gradients, you have both temperature and pressure covariant, but not perfectly. One of the things that happened early 2025 years ago when we all realized that this huge shake up in temperature was going to hit our bird communities was people predicted that with warming temperatures, birds would move upslope because they're going to track their temperature niche. So if they're adapted to temperature, they will move upslope. And people have looked for that a lot around the world and they've seen it in some cases, but in other cases it's hard to see or they they haven't seen it, or they've seen changes that are very subtle that seem to be lagging behind the rate of global temperature increase.

Chris Witt: When you consider the pressure part of elevation, you realize that there's a reason to predict that birds won't move upslope. There's a reason to predict they're going to stay in the exact same place. And when we think about these predictable hemoglobin changes, you realize that if they do move upslope, their hemoglobins are going to be mismatched with the elevation they're then at. So what we're getting is kind of this global, not a global escalator, but a global mismatch between temperature and pressure. And that's going to play out in really unpredictable ways. Some birds are going to move up, others will not. Some birds are, you know, they're, they're both physiological factors and also interspecies competition that are holding birds where they are in elevation or setting those elevation ranges. And we don't know how those are going to play out. Yeah, because the hemoglobin adaptations can be the basis for the competitive dynamics between two adjacent elevation replacement species.

Scott Taylor: OK.

Chris Witt: That's another thing we've looked at in our research. We've shown that competitive dynamics between hummingbird species can be sensitive to the oxygen pressure.

Scott Taylor: OK, cool.

Chris Witt: So that the low elevation species can be dominant when you have high oxygen pressure, when you reduce the oxygen pressure, the dominance can flip. And so we can assume that the gradient in oxygen pressure across elevation is one of the things that maintains the offset elevation ranges of those species. So if it gets warmer, we wouldn't predict that the low elevation species is going to move up in that case because it's probably maintained where it is, because it's got a superior competitor under low pressure conditions. That's just upslope from it.

Chris Witt: I had to show you this. This is a giant coot. And it's just such an anomaly. I had to show you because this, this bird makes no sense. There's still debate about whether it's flightless or flighted. They're all over the extremely high altitude lakes in the high Andes. It's a 5 lb coot. A normal coot is about 1/2 a pound. So this was like it's a 5 lb coot. It's enormous about, you know, 10 times the size of a regular coot. And they do really well on these super high altitude Andean lakes, like 4500 meters and up over 5000 meters. So they're supremely high altitude adapted them. We think about how the question of how altitude effects traits, well, sometimes it makes things bigger. In fact, if we look at all our Andean specimens, birds do get bigger at high altitudes. There's complex reasons for that, but the giant coot is just this awesome. You know, supreme example of gigantism at altitude.

Scott Taylor: Yeah. So from the birds you have in your collection, show us the smallest bird that's surviving at a high elevation.

Chris Witt: I'm not. Going to show you the smallest at the highest, but I'm going to show you the champion of torpor, which OK, great, the black metaltail.

Scott Taylor: OK.

Chris Witt: Thank you, Margie. This is a black metaltail. This particular one was at 3967 meters in elevation. At those elevations, every night this bird goes into deep torpor, which is something that hummingbirds can do that most other birds can't do. This species, the black metaltail, goes into the deepest torpor that's ever been measured. 8° C, meaning almost freezing on a night when it went to write about freezing. 8° C.

Scott Taylor: I think if folks want to learn more about torpor, which I assume they do because it's very fascinating. One of our upcoming episodes is OK, but do hummingbirds ever chill? Spoiler alert, the answer is yes. So what's the bird you have in the collection there that was collected at the highest elevation just overall?

Chris Witt: The highest bird I picked a species that probably goes higher or at least as high as any other, which is the white-winged diuca finch. And this is known to nest on glaciers. It's a glacier specialist. So we can we can predict right off the bat that this species is going to go extinct with climate change. It's either going to have to change completely what it does, or it's going to go extinct. This individual is collected at 4384 meters. Wow, which is probably pretty modest elevation for this species, which certainly goes above 5000 Wow. It's a tanager.

Scott Taylor: A tanager why do we common names are so confusing with birds? I mean Darwin tanagers are also tanagers, right? And for for US listeners who might not know meters, that's over 14,000 feet is where you're finding these these really amazing birds. And also for folks, when I use the word tanager, probably people are thinking scarlet tanager, western tanager, summer tanager. Those are not actually evolutionarily tanagers. They are cardinalids. And so, yeah, it's confusing, but the tanagers are this Neotropical radiation of birds that include Darwin's finches. And we'll get into maybe all of that and another episode, the confusing nature of common names from the birds behind you. Which one do you think is the coolest one in the collection?

Chris Witt: Well. Oh yeah, I see where you're going. There are many. Cool ones, but these are two species of tanagers, true tanagers, the ones that occur in the tropics. They're almost exclusively in the tropics, and these ones are classics, you know, Nice typical tanagers. They live in the canopy. They eat fruit, they also eat some insects. They're absolutely beautiful. They move around and mixed-species flocks. This one is the blue and black tanager, and it lives at pretty high elevations. For this particular type of sort of mixed species flock canopy tanager, it can get to about 3500 meters. Typically 2800 to 3300 is typical. If you go down a little bit, you get to the beryl-spangled tanager and that's sort of the mid, mid 2000s to low 2000s. So they overlap in elevation quite a bit. But these two of these two related species, this one tends to be higher and this one tends to be lower. Now when we were doing our hemoglobin studies, we were choosing species pairs that were related where one was higher than the other. We put blood of these two species into the analysis and we found that they're very striking differences in hemoglobin, in the oxygen affinity of hemoglobin between these two tanagers that are just offset in elevation a little bit and they actually are occurring at fairly modest elevations relative to the really high altitude specialists that we know in the Andes. So what these tanagers tell us is that hemoglobin adaptation is occurring at modest elevations. You don't need to be very, very high for it to happen. And there's a process of optimization of oxygen binding affinity to the elevation where you're at.

Scott Taylor: Yeah. We've reached the part of the show we call that's BS or that's bird stuff where we give our guests an opportunity to debunk a myth that ruffles their feathers. So Chris, what do you want to call BS on?

Chris Witt: OK, I've studied elevation for a long time and this is this is my pet peeve. People think that it's harder to go up in elevation and, and I get it because we're like a lowland species looking up and we know the challenge of moving up. But I want to, I want to introduce those people to an idea, which is that it's even harder to go down if you're a high elevation species.

Scott Taylor: Definitely, Yeah. And our own bias always leads us to think about the thing that's hard for us, not necessarily the thing that's harder for many more species than us on the planet. So that's a great BS.

Chris Witt: Thanks.

Scott Taylor: Well, thanks so much for joining us on the podcast today. It's been really fun to talk about how these birds make a living at high elevations. And yeah, I appreciate you taking the time.

Chris Witt: It's been a joy. Thank you, Scott.

Scott Taylor: Birds are dinosaurs, and around here we like our snacks. We end each episode with a dinosaur nugget. Today's nugget is some hummingbirds. The smallest warm-blooded flyers on the planet can keep hovering in air as thin as the summit of Mount Everest. They're burning around 10 times their resting energy just to hang in one spot in conditions that would put you or me on the floor in minutes. And the real altitude specialists go higher still. Meanwhile, we lowlanders with our oxygen cans and our training camps never evolved a single upgrade to the protein that carries oxygen in our blood, hemoglobin. Birds rewrote theirs over and over. So next time someone calls the hummingbird delicate, keep in mind some of them can handle thin air better than any human alive. That's a wrap on this week's episode. OK, but what's it like as a bird at the top of the world? Quick ask before you go. We're a small independent show, just two guys, so a single rating or follow does more for us than you'd think. One tap genuinely helps a new listener find us. So if you got something out of this one, leave us a rating or hit follow wherever you're listening. It takes 2 seconds and it makes a real difference. We'll catch you next time. Byeee.

This transcript was generated using AI-assisted transcription and may contain errors or omissions. Please refer to the audio or video episode for the most accurate representation.