The Science of Birds - How Birds Fly

Episode Date: October 20, 2021

Birds, probably more than any other aerial creatures, have amazed and inspired us with the grace and power of their flight. So just how do they do it?In this episode, we'll look at the physics and anatomy of bird flight.I’ll start off with the basics of aerodynamics as it relates to bird flight. That’s the meat and potatoes of our lesson today. But we’ll also consider the different ways that birds fly—their different modes of flight. Last, we’ll examine some additional adaptations birds have that make them high-octane flying machines.~~ Leave me a review using Podchaser ~~‍Links of InterestBird Flight [Wikipedia]Why do birds fly in a V? Endangered ibis reveals its amazing secretThe amazing muscles and bones that make birds flyLink to this episode on the Science of Birds websiteSupport the show

The Earth's atmosphere is an incredibly dynamic environment. It's made of mostly nitrogen and oxygen gases, which are, of course, invisible to the human eye. Even though we can't always see what's going on in the atmosphere, we know air masses are in constant motion. We feel some of these movements as wind. In the most extreme cases, we get hurricanes and tornadoes. There are also colossal, undulating rivers of air like the sea. the jet stream. And warm air rises high into the sky as billowing columns we call thermals. In other places, masses of air sink back down to earth, piling up to form high-pressure areas.

There's a lot of activity in the atmosphere. It's a vast and wild place. Several groups of animals have independently evolved the ability to fly, to brave this aerial frontier. First, it was the insects, then the flying reptiles known as pterosaurs, then birds, and finally bats. These animals all fly under their own power. Well, in the case of pterosaurs, they flew under their own power, since those guys are all extinct. And I guess you could say that one primate, in the form of Homo sapiens, managed to get into the sky as well. But we cheated and took a shortcut, didn't we? We got up there using science and technology, through the process of cultural rather than biological evolution.

Flight has evolved more than once in the history of life because, presumably, it offers some major advantages. If nothing else, flying is often the most energy-efficient way for an animal to get from point A to B. To cover one kilometer, for example, a bird will use up only 1% of the calories that a mouse of the same size would burn running that same distance. However, and this is a big, however, flying requires an enormous amount of energy, in general, if you look at it moment by moment. Getting off the ground is hard and takes loads of energy. Staying airborne is hard too. Each group of flying animals has amazing adaptations to deal with these challenges. After millions of years of evolution, they're so well adapted for flight that

they make it look easy. Birds, probably more than any other aerial creatures, have amazed and inspired us with the grace and power of their flight. So just how do they do it? Hello and welcome. This is the Science of Birds. I am your host, Ivan Philipson. The Science of Birds podcast is a lighthearted, guided exploration of bird biology for lifelong learners. This episode is an overview of how birds. birds fly. Now, maybe the average Joe, walking down the street, thinks he's got it all figured

out. How do birds fly? Well, they just flap their wings and up they go. Simple as that. The end. No need for Mr. Fancy podcast host to blabber for an hour to explain all that to me. Yeah, sorry average Joe, but as usual, nature is far more complex than what we might assume. Avian flight involves a lot more than just wings go flappy-flappy and then bird goes up, up, and away. So let's look more deeply into how birds take to the air and how they travel through the sky. Let's see if we can get our heads around how this really works. This topic is complex enough that if we're not careful, we could get bogged down by explanations of physics equations. Let me see a show of hands for how many of you want to listen to me describe physics equations.

No one? Really? Well, suit yourselves. I'll do my best to keep the discussion reasonably simple. But I warn you that there's no way to entirely avoid some technical mumbo-jumbo. And if you happen to be a physicist listening to this, there's a chance you'll be shaking your head with a furrowed brow at some point here. Sorry, I'm just doing my best as a very non-physicist. I'll start off with the basics of aerodynamics as they relate to bird flight. That's the meat and potatoes of our lesson today. But we'll also consider the different ways that birds fly. They're different modes of flight. Last, we'll examine some additional awesome adaptations birds have that make them high-octane flying machines.

As I said, flying is hard. It requires lots of energy. Each aspect of flight has its challenges, from taking off to maneuvering, to keeping stable and landing. Birds meet these challenges, in the broad sense, by having aerodynamic, streamlined wings and bodies, and by using energy as efficiently as possible.

And these things are related, right? The more aerodynamic you are, the less energy you need if you're going to fly, generally speaking. To get off the ground and stay aloft, a bird needs to overcome two major physical forces, gravity and drag. Drag is what we sometimes call air resistance. To deal with gravity and drag, the bird generates its own physical forces, lift and thrust.

It uses thrust to overcome drag, to propel itself through the air, and the bird uses lift to defy gravity. It's funny that we always say defy gravity. Why not disobey gravity or mock gravity? You don't hear documentary narrators saying, The bird takes to the air in mocking disobedience of gravity. As if to say, screw you, gravity, do not tell me what to do. You're not my dad. So there are these four forces acting on the body of a flying bird. Gravity, drag, lift, and

thrust, thrust, offsets gravity, thrust offsets drag. Lift is generated by the way moving air interacts with the shapes of the bird's wings, body, and tail, mostly the wings, of course. So let's go ahead and take a look at a bird's wing. Imagine you're looking at a generic bird from the side, as it simply glides past you without flapping. It's just holding its wings out fully extended. It rudely ignores you, refusing to acknowledge your existence. And you know, when I say generic bird, what comes to your mind, I wonder? Maybe a house sparrow or a pigeon, an American robin, perhaps a dunnick or some kind of blackbird? For me, I imagine a gull when I think of a generic bird. For some reason,

who knows? Anyway, if we take a cross-section of our generic bird's wing, about halfway between the tip and the shoulder, we see it has a special shape. The part that faces into the wind, the front of the wing, is thicker and rounded. By contrast, the trailing edge is thin and tapered. The wing in cross-section like this has a sort of teardrop shape, but turned on its side and stretched out. This shape has a name. We call it an airfoil. Airplane wings also have this airfoil shape. The airfoil is so fundamental that we see it in pretty much everything that flies or glides. Bat wings, dragonfly wings, flying squirrels, helicopter blades, boomerangs, and the seeds of maple trees. to name a few. If a bird just had flat wings without an airfoil shape, like stiff rectangles of

cardboard, say, it wouldn't get off the ground, no matter how hard or fast it flapped. Now there's a little more detail that I need to describe here. The airfoil shape of a bird wing has more curvature on the top surface compared to the bottom. In other words, it's more convex on top. This is really important for generating lift. The wing generates lift because air flows faster over the curved top of the wing than across the relatively flat bottom.

This difference in air speed causes there to be lower pressure above the wing and higher pressure below the wing. And that higher pressure pushes the wing upward. That upward push, that force is lift.

Lift is generated in the direction perpendicular to how the air is flowing across the wing. So if the wing is held out perfectly flat, in a horizontal plane, and the air is flowing across it, the lift in that case will be straight upwards. But the bird probably wants to move forward too,

not just up. This is where thrust comes in. To move forward, a bird needs to generate thrust. Airplanes generate thrust using propellers or jets. But the last time I checked, no bird has propellers. So birds have to use their wings for both lift and thrust. That's what flapping is for. On the downstroke of a single flap, a bird rotates its wing so that the front end is lower than the back end. Remember that I said lift acting on the wing is perpendicular to the

direction of airflow? Well, when a bird rotates its wing like this on the downstroke, some of the overall lift gets transferred to the forward direction. It becomes thrust. When a bird rotates its wing like this, we say that it changes the angle of attack. That's the angle that the wing quote unquote attacks the air or slices into it. If you follow the movement of a wing over the course of a single flap or stroke, you'll see that the angle of attack changes at several key moments.

Most of a bird's lift and thrust are generated only by the downstroke. But some birds, especially hummingbirds, gain lift and thrust on the upstroke as well. It all depends on the angle of attack. And you might imagine that if a bird wants to fly faster, it just needs to flap faster. That's generally not the case, however. To fly faster, a bird deepens the angle of attack on the downstroke, and it increases the amplitude of the downstroke. So it flaps harder, not faster, to get more thrust.

That, dear listener, is the basics of lift. Birds use lift from their wings to disobey gravity. The heavier a bird, the more lift it needs. Birds also flap their wings to generate thrust to propel themselves forward. But there's still the problem of air resistance, right? The force we call drag? There are a couple flavors of drag. involved in bird flight. First, there's the drag that's a form of friction. Air moving over

the bird's wings and body creates friction that slows the bird down. We call this profile drag. The faster a bird flies, the stronger profile drag becomes. Flapping harder, more forcefully, is one way to overcome profile drag. But that takes more energy, doesn't it? A better way for a bird to reduce this sort of friction slash drag is to have a highly streamlined body. A contoured body outline and smooth feathers go a long way to reducing profile drag. Another form of drag results from the force of the wing moving and displacing the surrounding air. This is called induced drag. It slows a bird down as well.

Air pressure differences around the wing can create a little vortex in the air, trailing behind the wing. This effect is strongest at the wing tips. The vortex is what causes induced drag. Well, that's my understanding anyway, that the vortex causes the drag and not the other way around. The wing tips and the tips of the tail leave swirling vortices in the bird's wake.

Huh, it's not often that we get a good excuse to use the plural form of the word vortex. Vortices. Fun word, right? We'll return to these vortices when we talk about geese flying in V formations a little later. Unlike profile drag, induced drag decreases as flight speed increases. So the two forms of drag respond in opposite ways to how fast a bird is flying. Why does this matter?

Well, this all relates to how much energy a bird needs to maintain flight at different speeds. At slow flying speeds, induced drag is high. There's a lot of vortex action at slow speeds, so it takes a lot of energy for a bird to maintain enough lift and forward movement when flying slowly. At fast speeds, on the other hand, profile drag dominates, and it becomes the primary limiting factor. But there's a sweet spot in the middle, an optimum speed.

At intermediate flight speeds, there's a balance between profile drag and induced drag, such that the energy needed to fly is at a minimum. This is when drag overall is as low as possible. This optimum speed actually varies a bit, depending on the needs of the bird. One sweet spot is great for using the least amount of energy minute by minute. That's a good speed for aerial foraging birds like swifts as they zip around catching insects on the wing. There's another sweet spot at a slightly higher speed for a bird that needs to burn the fewest calories to cover a given distance. Long-distance migrants like waterfowl or shore

birds will tend to fly at this optimum speed. And I should point out that there are no universals for these optimum flight speeds. It's not like 42 miles per hour is the sweet spot for all birds. Instead, each species is adapted to fly most efficiently at its own optimum speed. This is all a simplification, of course, but I hope you get the ideas. I'm trying and perhaps failing to keep this from getting too technical. Oh well, I guess the word science is in the podcast title. And hey, you're still with me, which is fantastic. So let's move on to looking at the shapes of bird wings. All flying birds have wings with the basic airfoil shape I described earlier.

But other than that, there's a lot of variation in wing shape across the avian world. For example, some types of birds have long, narrow wings. Others have short, broad wings. Each wing shape represents a balance between a bird's particular needs for speed, maneuverability, and energy efficiency. So one size definitely does not fit all. The shape of a given species wing is tailored to how and where the bird lives. Two important concepts here are aspect ratio and wing loading.

Aspect ratio is the length of the wing relative to its width. Length, in this case, is the distance from the wing tip to the shoulder. When we say a wing has a high aspect ratio, for example, that describes a long wing that is relatively narrow. The other value, wing loading, is calculated as the weight of the bird divided by its total wing area, area as in how many square inches. My backyard chickens, for example, have chunky bodies and pretty stubby wings. In other words, a lot of weight combined with a small wing area. Therefore, their wing loading is high. Birds with high wing loading, like domestic chickens, have a lot less lift than birds with low wing loading. So I guess my hens won't be soaring

through the clouds anytime soon. We can describe the wing aspect ratio and wing loading of any given bird. But there are four basic bird wing types. These are elliptical, high speed, active soaring and passive soaring. Something elliptical is oval-shaped, right? The elliptical wing is short, broad, and rounded at the tip. We find this wing type in sparrows, crows, robins, warblers, grouse, and many other birds. The aspect ratio of an elliptical wing is low by definition. Wing loading, however, varies among birds with elliptical wings. Warblers have large wing areas combined with a low body weight. So flappy little songbirds like this tend to have low wing loading. Grouse, on the other hand, are more like my pet chickens. They have high

wing loading. Grouse and similar heavy-bodied ground-dwelling birds usually only fly short distances to escape predators and whatnot. The elliptical wing type is excellent for fast take-off, short bursts of speed, and high maneuverability. You can imagine the These features would be useful for escaping a pouncing predator. Elyptical wings are also handy for flying in a forest, where a bird needs to dart through tree branches. The next wing type is high speed. These wings are of medium to longish length and somewhat narrow.

Importantly, they taper to fine points at the tips. Think falcons, sandpipers, swifts, ducks, and turns. The wings of such birds are good for more sustained periods of fast flight. Their special shape minimizes profile drag, which, you may recall, increases as flight speed increases. The third type is active soaring wings. These are simultaneously long and narrow. They're what we find on albatrosses, sheer waters, gannets, gulls, and the like. These birds have high aspect ratio wings combined with,

low wing-loadings. This maximizes their lift, especially in the wind. You may have noticed that the birds with this wing type tend to be seabirds. Their wings allow them to take advantage of wind blowing over the ocean, sweeping up the faces of sea cliffs, or whipping over the crests of waves. The last of the four major wing types is passive soaring. These wings have low aspect ratios, which means they are broad relative to their length. Picture the wing of an eagle, a condor, or a stork. Birds with this wing type are often species that fly around over land rather than the ocean. They also tend to have low wing loading.

Passive soaring wings are well adapted for taking advantage of rising air masses like thermals. Birds such as eagles and vultures can gain altitude passively by riding a thermal, by just holding their wings out. They barely need to flap. A key feature of passive soaring wings is the slots in the wing tips. You know, when you see the primary feathers at the tip of an eagle's wing

spread out like fingers? There's actually a super important function for that. Remember that there are those little vortices that form at the tips of the wing. They cause induced drag. This is especially problematic at the slow flight speeds typical of a soaring bird like a.

eagle. The slots in the passive soaring wing break up the vortices and therefore cut down on induced drag. So those are the basic wing shapes, elliptical, high speed, active soaring and passive soaring. But unlike an airplane, an individual bird can change the shape of its wings from one instant to the next. It has about 50 muscles that control its wing, giving it a wide range of motion for adjustment. An individual bird changes the posture and angle of its wings depending on what it needs at the moment. One posture is ideal for flying as fast as possible,

another is best for gliding, and so on. And also, unlike an airplane, a bird can move its two wings independently. This adds even more agility and allows for some amazing aerial acrobatics. Birds can turn on a dime, fly upside down, stop short by stalling, and do barrel rolls. Do a barrel roll! Besides being able to change their wing shape as needed, most birds have some options for how to fly. There are several modes of flight.

We have gliding, soaring, flapping and intermittent. This is sort of like how people can walk, jog, run, and skip. Gliding is perhaps the simplest mode of flight. This is where a bird has its wings fully extended and held steady without flapping. All of those interacting aerodynamic forces keep the bird up in the sky, at least for a while anyway. If the bird doesn't flap, it will slowly descend, gliding down as gravity and drag take their revenge for being disobeyed. Soaring is what happens when a gliding bird catches a ride on a moving mass of air. In thermal soaring, the air mass is a rising column of warm air.

This is what we often see with eagles, hawks, vultures, etc. Dynamic soaring is a bit more complex. This is where the bird rides the wind, wind whipping up over a mountain ridge, for example, or over the crest of a wave on the ocean. Wind provides a free ride with lots of lift. So gliding and soaring are energy-efficient ways of travel. Since there's very little flapping involved, birds using these modes of flight burn less energy.

We've already been talking about flapping flight. There are definitely many more nuances we could get into regarding flapping, but there's just no time, I tell you, we have to move on. So this brings us to the intermittent flight mode. You've probably noticed that birds don't always just flap nonstop as they fly around. There are a couple of forms of what we call intermittent flight. There's flap gliding and flap bounding. Flap gliding is pretty self-explanatory.

This is where a bird flaps a few times, then glides for a bit, then does a few more flaps. flap bounding is different in that in between bursts of flapping the bird folds its wings in tight to its body fold it up like this the bird becomes like a little bullet its body is aerodynamic to an extent but it doesn't have much lift once the wings are pulled into the body the bird starts falling we see flap bounding in many birds including finches sparrows and woodpeckers Watch one of these birds and you'll see that it has an undulating flight path. It rises up when it flaps and sinks when it goes into bullet mode. It's bounding, up and down, up and down.

Flap gliding is best suited for slower flight speeds, while flap bounding works better at higher speeds. Both of these intermittent flight modes work to save energy. Anytime you don't have to flap, you save energy. Now I have a few more factoids to drop on you regarding flight modes. Besides gliding, soaring, flapping, and intermittent flight, there's hovering. Hovering is amazing and rare among birds. It's rare probably because it takes a ton of energy to pull off.

Hummingbirds are the masters of hovering, of course. As I mentioned, they gain lift on both the downstroke and upstroke. They have incredible control over their lift and thrust. One reason these tiny birds can hover so much is because, well, they weigh hardly anything. And, importantly, their food is super high in energy. Flower nectar is like jet fuel for these guys. Other than hummingbirds, we don't see many other birds hovering for more than brief moments. The largest non-hummer that can do some sustained hovering is the pied kingfisher,

Surrilli rudis. Then we have the phenomenon of birds flying in formation. The classic example is the V formation of geese during migration. This brings us back, once again, to those vortices that form at the wingtips. If you're a goose flying right behind another goose, well, first off, your view isn't going to be that great. Do you really want to fly thousands of miles staring at a goose caboose? Second, the swirling air from the wings of the goose in front, from those vortices, is going to cause extra drag on your wings. Extra drag means you'll have to burn more energy.

No bueno. The solution is to fly off to the side, but still behind the other goose. Not only is your view vastly improved, but now you can actually benefit from the vortex action of the other bird. This is because if you position yourself just right, you can catch the swirling air as it comes up underneath your wings. It makes flying a bit easier for you. The same is true for the goose behind you, flying off to your side. Birds like geese and pelicans flying in V formations can save 10% of their energy or more on long flights. Did I mention that flying is hard?

Because it is. It demands a lot from the body and brain of a bird. There are few anatomical features in birds that are not adaptations to a life on the wing. Entire books have been written about how well-adapted birds are for flying, but let's consider at least a few more adaptations here, besides wings. Some of these adaptations help reduce a bird's weight, some improve its aerodynamics, and others help it use energy more efficiently.

Over their long evolutionary history, birds have lightened their loads in multiple ways. The less you weigh, the easier it's going to be for you to fly. A bird's skeleton has some fascinating features for reducing weight, while still maintaining strength. You've heard that birds have, quote-unquote, hollow bones. More technically, we say that birds have pneumatic bones. There's a silent P in there.

It's spelled P-N-E-U-M-A-T-I-C. Having pneumatic bones saves a lot of weight, for sure. But believe it or not, a bird's total skeleton doesn't necessarily weigh less than a skeleton of a similarly sized mammal. That's because some parts of a bird skeleton are actually beefed up, thicker here and there to provide extra strength. so it can all sort of balance out. In any case, bird skeletons also show other adaptations for weight-saving and strengthening. There are multiple parts of the avian skeleton where bones have become fused. Meanwhile, some bones have been reduced in size and others have been lost completely.

For example, the loss of the tail, teeth, and heavy jaw bones are all adaptations for lightening the load. A special feature of the skeleton is the keel, which is a conspicuous extension of the breastbone. At least for birds that fly, the keel is huge. It provides the attachment points for the massive flight muscles. These are the pectoral and suprachorochoidious muscles. You've heard of the pectoral muscles, no doubt, but what about the suprachorochoideus? That's a mouthful, isn't it? I mean, it might as well be supracorrochoideistic expialadocious.

I mean, even though the sound of it is something quite atrocious. This muscle, the suprachorochoidious, is part of a bird's breast. But when it contracts, it pulls the wing upward, using an ingenious pulley system with a tendon that runs through the shoulder. So the supracoricoidious is responsible for the upstroke. In a non-flying animal, you might expect such a muscle to be on the back. That's how it works for humans. but a flying bird needs to have a lower center of gravity to maintain its aerodynamic form.

Thus, the muscles used for the downstroke and the upstroke sit firmly in the breast region. Another adaptation for flight that reduces weight is the way that a bird's gonads and some other organs change size over the seasons. The gonads tend to be tiny for most of the year. They might as well shrink to save weight when they aren't needed. A similar thing happens to some organs during migration. For example, in some shorebirds that make long-haul migrations, their digestive systems temporarily shrink during migration. The stomach and intestines atrophy when the bird isn't eating much.

There is one other feature of the wing that I should mention, the Alula, A-L-U-L-A. Alula means winglet in Latin. This is a bird's thumb. Several small flight feathers are attached to the thumbbone. Most of the time, these feathers are pressed close to the wing and we don't really notice them. But the alula has an important function. It improves a bird's aerodynamics when flying at slow speed or landing.

When the allular feathers are raised, a small slot forms between the alula and the rest of the wing. Air passing through this slot creates a vortex, which somehow helps to keep the bird from stalling. So look for the alulas the next time you watch a bird land on a perch. These little winglets make the bird look like it's giving you two thumbs up. So not only will you see this cool

anatomical adaptation, you'll get a boost of confidence from the approval of a bird. And side note, I just put a lula on my list of baby names in case I ever have a daughter. Now, what about adaptations that improve energy efficiency. One of the most remarkable things here, in my opinion, is the avian respiratory

system. It's insanely efficient to maximize the oxygen absorbed into the blood. It's so complex and fascinating that I should probably dedicate an entire episode to the respiratory system of birds. But the basic idea is that the air a bird inhales travels a one-way circuit through the system, traveling through it for two complete breath cycles. This is accomplished by the lungs, air sacks, and a series of tubes. You know, a series of tubes like the internet. A series of tubes. This respiratory system works together with other organs, like those of the circulatory and digestive systems, to give birds a supercharged metabolism. Birds can digest and process food quickly to get the enormous amount of energy they need for flight. The fundamental

source of food energy for humans and most other mammals is carbohydrates like glucose. Birds, however, use fat as their go-to fuel source. Oounce for ounce, fat contains more than twice the number of calories that carbs do. So by depending more on fats, birds get the most bang for their buck. They get way more energy from their food, but without adding any weight. It seems that being able to fly, despite the challenges, is a big advantage in the great game of life on Earth. Looking at the entire animal kingdom, we find that those few groups that have mastered flight are among the most diverse on the planet.

There are at least 900,000 insect species out there. Bats with 1,400 species represent 20% of all the mammal diversity. And there are about 11,000 bird species. Long before the other dinosaurs went extinct, the earliest birds left the ground when they evolved the incredible ability of powered flight. They took to the skies to swim through all that invisible oxygen and nitrogen. The dynamic atmosphere, with its endlessly swirling air masses, became the wild frontier for birds. Powered flight opened up vast amounts of space to them.

Flight gave birds the ability to travel from one side of the planet to the other in just weeks or even days. It allowed them to take advantage of new niches and of endless opportunities. I really hope you enjoyed this episode on How Birds Fly. It was a little tricky for me to write this episode because I had to rely on words to explain concepts that are best explained with the help of images. I'll put a couple links in the show notes to some diagrams that illustrate some of the ideas we discussed today. In any case, thank you very much for taking the time to learn about birds with me today. Here's a loud and passionate thank you to my awesome supporters on Patreon, who are helping to make this podcast possible. And of course I want to give a special shout out to my newest patrons, Laura Barton, Christina Sells, Lauren Matheson, and the Piwani family.

Thank you so much for your help. It really means a lot. If you, lovely person who is listening right now, are interested in supporting this podcast as a patron, you can check out my Patreon page at patreon.com forward slash science of birds. As always, if you have something you'd like to share with me about the podcast, or what you think of as a generic bird, go ahead and shoot me an email. The address is Ivan at Scienceofbirds.com. The show notes for this episode, which is number 37,

can be viewed over on my website, scienceofbirds.com. This is Ivan Philipson, and I'll catch you next time. Peace.