Bat Flight Versus Bird Flight

Bats are better at flying than birds are.

I can hear all you bird fanatics out there revving your engines, but that’s not just my personal opinion (ok, it mostly is). Still, there are some sound scientific facts in there.

Look, if you compare the effortless soaring of something like a turkey vulture to the frenetic flapping of a little brown bat, of course the bird is going to seem better off. But most people haven’t spent a lot of time around bats, or watching bats fly. Because they generally fly at night.

I spent a summer mist-netting for bats in rural Ohio once, and let me tell you, your opinion on bat flight prowess changes after you have seen a bat fly within a few centimeters of a vertical wall of net and completely reverse direction in midair. All in a split second.

Bats are really, really good at flying.

We still managed to catch some because, as my superior put it, "They get lazy about echolocating sometimes." I suppose I'd get tired of having to scream in order to see too.

But we still managed to net some because, according to my supervisor, “they get lazy with their sonar.” (The bats were not harmed, just irritated.)

There have been only four major groups of animals that have achieved liftoff, and they all achieved it quite differently. The first is the insects, of course, which are among some of the most successful and diverse fliers out there- from dragonflies to butterflies to actual flies. The remainder are among the vertebrates: the pterosaurs, the birds, and of course, the bats. (Arguably, humans too, if you count building flying metal thingies.)

All other “flying” animals you may have heard of, such as flying squirrels, are actually gliders.

We caught THEM in our nets as well.

We caught THEM in our nets as well. You may not know this, but flying squirrels are incredibly evil and they bite a million times harder than bats do.

The difference between gliding and flight is, very simply, that a flying animal is able to generate thrust. This is why gliders generally hold themselves stiff to maximize their surface area- they don’t have the power to generate it. Fliers do via all that flapping.

Ok, so I’m gonna attempt to do a very, very quick breakdown of basic powered flight mechanics and terminology. Bear with me here.

In order to fly, you need to a) get up into the air and b) stay in the air without falling. Gliders, of course, are falling after they get into the air, unless they happen upon a lucky gust of wind. They’re just falling with style.

Birds, bats, and flying insects (and airplanes) employ what is called an airfoil to help them generate lift- i.e., upward motion. An airfoil is a wing shape that is more sharply curved on the top than the bottom. It looks something like this.

1022px-Wing_profile_nomenclature.svgWhen an airfoil is put with its thicker surface into the wind and angled upwards, something peculiar happens. Due to the sharp curve of the top surface compared to the bottom surface, the air moves faster over the airfoil than under it. This creates less pressure over the top of the wing than underneath it- in other words, it subverts gravity!

That’s how airfoils generate lift. But then why all the flapping? Well, in order for an airfoil to work, you’ve got to have air constantly flowing over the thicker surface. This is called thrust and flapping is what moves that air. Airplanes, of course, achieve thrust with their engines, which is why they don’t have to flap their wings.

There are two main wing motions in animal flight: upstroke and downstroke. The downstroke is what generates thrust by forcefully pushing the airfoil surface through the air. The animal maximizes the surface area of the wing pushing down against the air in this motion. Pushing the wings closer together underneath the body also creates more air pressure that provides additional lift. The upstroke is used to maneuver the wing back into position for the next downstroke, and animals usually minimize the surface area of their wing pushing against the air (to reduce their chances of generating backwards thrust!)

Pigeons are shown here generating both thrust and lift to get up into the air. The thicker part of their wing surrounding bone and muscle is what forms the shape of the airfoil. (Photo by Toby Hudson.)

Pigeons are shown here generating both thrust and lift to get up into the air. The thicker part of their wing surrounding bone and muscle is what forms the shape of the airfoil. (Photo by Toby Hudson.)

Birds and bats both use these mechanics to stay in the air. But it’s the different ways that they approach them that makes it so fascinating.

When you look at videos of birds and bats flying, there aren’t many immediately apparent differences between the motions. In fact, for a long time researchers thought that they flew in essentially the same way: using their wings like rotating paddles.

Anybody who’s ever swum the butterfly stroke will be familiar with the motion: more or less throwing your arms forward to drag the water (or air) behind yourself. Take a moment to think about doing that, and then appreciate the sheer force that bird and bat muscles have to be able to generate.

However, while this is more or less how birds fly, it turns out that bats fly with a much more complex range of motions. They’re just a lot harder to see.

Consider the anatomical differences between bat and bird wings. The first is obvious: birds have feathers, while bats’ wings consist largely of skin membranes. The second is also pretty obvious: bird wings consist of elongated arms and a single finger, while bats use three fingers with the membrane stretched between them. Because the end shape of both is kind of similar, it’s easy to miss how large this difference actually is.

Jh1myIIf you look closely at the above gif, you’ll notice that at several points during flight, the bat actually bends its fingers, which dramatically changes the shape of its wings. Birds do not have joints in their feathers, so they cannot do this. The huge amount of flexibility in bat finger joints is even more apparent when you look back at that picture above of a bat caught in a net- where the wings may appear crumpled are actually where the bat bent down its fingers.

Bats can bend their fingers to the point where their entire wings seem to disappear.

Bats can bend their fingers to the point where their entire wings seem to disappear.

Here's a better look at where all those little fingers get tucked away.

Here’s a better look at where all those little fingers get tucked away.

Flexible joints are not all the bat has in its arsenal. Its actual bones are flexible, due to a lack of calcium in its diet. This means that they deform and reform their shape during flight.

Birds minimize air resistance by rotating their primaries during their upstroke, allowing air to slip between the feathers. Bats, with solid membranes, can’t do this- so they have an even finer means of control. There are lines of muscle present within the bat’s wing membrane that can actually change the stiffness and malleability of its skin. You can see them quite clearly under the skin of our entangled bat friend.

This is a big brown bat (Eptisicus fuscus) by the way.

This is a big brown bat (Eptesicus fuscus), by the way.

These muscles allow the bat to make their membranes flexible during their upstroke to decrease resistance, yet stiff during their downstroke in order to provide lift. It also allows them to change the camber (angle) of their wings on a whim!

This slow-mo video really displays just how incredibly flexible bat wings are.

Bat wings are also covered by millions of tiny, hyper-sensitive hairs that allow the bat to sense air currents and adjust accordingly.

So what does all this control do for the bat?

Well, for one thing, it means they’re not limited by symmetry. Bird wings will almost always mirror each other in shape, while bats may form two different wings shapes at the same time, allowing them to perform some crazy aerial acrobatics. Some insect-eating bats will actually grab an insect by wrapping one wing around it midflight (don’t believe me? You can see it in the beginning of this video!) and then get the insect in their mouth all in a split second, while still flying.

Now, in terms of speed, birds can generally outpace bats. But in terms of maneuverability, bats can fly circles around birds.

The fact that bats’ bones, unlike those of birds, aren’t hollow, and that their skin is heavier than feathers might seem like a disadvantage- but it isn’t. Birds have much more mass in the center of their body than they do in their wings; by contrast, bats have more mass distributed through each wing (12-20% per wing). This means that bats can actually push off their own mass do do things like flip, spin, roll, etc. No bird can stop midflight and flip over to land upside-down, but bats can.

Because they have such fine control over their airfoil shape, bats can also generate lift using less energy than birds. Remember when I talked about minimizing surface area during the upstroke and maximizing it during the downstroke? Bats can bend their fingers and ‘crumple’ their wings as they raise them, conserving energy. Think of it like opening and closing an umbrella. While birds can pull their feathers together more tightly, they can’t exactly clench them like fists.

Decreasing energy costs is good in any situation, but particularly for fliers. It takes a lot of energy to fly. In this case, bats can outcompete both birds and insects for energy efficiency- one study found that nectar-feeding bats, though the largest in size, expended the least energy hovering when compared to both moths and hummingbirds.

So: bats can do all these cool things while flying. Is there a downside? Mmm… yeah. Besides sheer speed, birds do have one major ability that bats don’t: they can walk.

Ok, some bats can walk (four species) and some birds can’t (hummingbirds), but for the most part birds are fairly okay at waddling around on the ground. Most bats… not so much.

We can draw parallels between hummingbirds and bats in this respect, as hummingbirds are among the most maneuverable of all bird species. Selection for increased acrobatics in the air seems to come at the cost of land maneuverability. It makes sense.

And unlike birds, most bats can’t take off directly from the ground. It’s that pesky quadrupedal stance they have. Birds, being bipedal, can push off with their hind legs. Bats can’t do this, aside from a few exceptionally powerful species like the vampire bats and the eastern red bat. Generally if they land on the ground they have to climb up to a vantage point, like a tree trunk, before pushing off.

Interestingly, the other vertebrate fliers- pterosaurs- were also quadrupedal, but could push off from the ground. They used their forearms!

Anyhow, despite their land-based limitations, bat flight is simply incredible. I encourage you to go out on a summer night just after sunset and look to the skies. Maybe you’ll get to witness it for yourself!

Read on: I’ve written more on bats- an overview of bat diversity, a post on species called ghost bats, and a species overview of one of the few walking bats, the New Zealand pekapeka! To view a list of all my nonfiction articles, check the Nonfiction page.

Links

Bat vs Bird Flight

Bat Flight Research Program

References

Hedenström, A., Johansson, L. C., Wolf, M., Von Busse, R., Winter, Y., & Spedding, G. R. (2007). Bat flight generates complex aerodynamic tracks. Science, 316(5826), 894-897.

Hedenström, A., Johansson, L. C., & Spedding, G. R. (2009). Bird or bat: comparing airframe design and flight performance. Bioinspiration & biomimetics, 4(1), 015001.

Riskin, D. K., Bahlman, J. W., Hubel, T. Y., Ratcliffe, J. M., Kunz, T. H., & Swartz, S. M. (2009). Bats go head-under-heels: the biomechanics of landing on a ceiling. Journal of Experimental Biology, 212(7), 945-953.

Riskin, D. K., Bergou, A., Breuer, K. S., & Swartz, S. M. (2012). Upstroke wing flexion and the inertial cost of bat flight. Proceedings of the Royal Society B: Biological Sciences, rspb20120346.

Swartz, S. M., Iriarte-Diaz, J., Riskin, D. K., Song, A., Tian, X., Willis, D. J., & Breuer, K. S. (2007). Wing structure and the aerodynamic basis of flight in bats. AIAA J, 1.

Tian, X., Iriarte-Diaz, J., Middleton, K., Galvao, R., Israeli, E., Roemer, A., … & Breuer, K. (2006). Direct measurements of the kinematics and dynamics of bat flight. Bioinspiration & Biomimetics, 1(4), S10.

Voigt, C. C., & Winter, Y. (1999). Energetic cost of hovering flight in nectar-feeding bats (Phyllostomidae: Glossophaginae) and its scaling in moths, birds and bats. Journal of Comparative Physiology B, 169(1), 38-48.

About Koryos

Writer, ethology enthusiast, axolotl herder. Might possibly just be a Lasiurus cinereus that types with its thumbs.
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2 Comments

  1. sreeja mukherjee

    this was really helpful. got to know so many things. even our textbooks didnt mention these points except d 1st point which we al know 😛 🙂

  2. The differences between birds and bats are much more than what a layman like me thinks of. I feel thankful to the author for the very interesting information.

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