Why are there no more large flying birds?

February 6, 2014 • 6:33 am

by Matthew Cobb

As is well known, Professor Ceiling Cat can’t be doing with Tw*tter. Here’s yet another example of why he’s wrong, and should learn that that micro-bl*gging site is not just for knowing what celebrities had for breakfast or for launching cyber lynch mobs.

I was listening to Radio 4’s ‘Tweet of the Day’ this morning at 05:58. It featured the bizarre call of the Great Bustard (it sounds roughly like someone blowing their nose and farting at the same time). The Great Bustard is a large bird that was hunted to extinction in the UK, but has recently been reintroduced and is now successfully breeding. Chris Packham, who did the commentary, claimed that at 16 kg the Great Bustard is one of the heaviest extant flying birds.

This struck me – 16 kg isn’t much. Is this an absolute limit to flying? What about those pterosaurs – some of them were HUGE. How come they got so big and flying birds don’t? What’s the upper limit on the weight of a flying animal?

So I got out my iPad and tweeted @TetZoo aka Darren Naish, who knows about all things tetrapod. (I got the weight wrong. It was early in the morning. This caused some confusion, as you’ll see.)


Both Darren and Dave Hone, another pterosaur expert chipped intw3




The ‘different take-off’ caught my eye. I know there’s been a suggestion that pterosaurs lived on cliffs, so could simply soar without having to take off from the ground (the modern swift, hardly a chunky bird, can’t take off from the ground). But some pterosaurs would dive and eat fish – how did they take off from the sea?:


Dave replied:


Darren had to set the record straight regarding bustard weight, when David Watson rightly questioned my figure:tw10tw13


Then Mike Habib joined in and pointed out:


Then he asked the Big Questiontw8

Tommy Leung chipped in:

[JAC comment: Why is Habib so sure that “birds and bats can’t get giant pterosaur size”?]

So, as in most interesting questions, the answer to ‘Why are there no large flying birds now’ appears to be ‘We don’t know’.

Any ideas?

[JAC comment 2: I doubt this demonstrates that I’m wrong about Tw**ter. All that scientific brainpower results in the verdict that “we don’t know”?? They might as well have tw**ted what they had for breakfast!]

Links: Dave and Mike’s piece on how pterosaurs took off, the PLoS One paper from Mark Witton and Mike Habib, looking at whether giant pterosaurs could fly, cited by Darren.

89 thoughts on “Why are there no more large flying birds?

    1. No, the only time there was a substantial change in atmospheric compilation over the last 500 myr or so was during the Carboniferous, long before the pterosaurs and even longer before the birds. Increased oxygen levels, it is argued, led to large terrestrial arthropods during the period.

  1. Well Jerry, part of the lack of answer is struggling to even explain what the issues are in 140 characters, let alone point to papers or get into the details.

    While I’m not a fraction into the maths or mechanics as Mike Habib, he has pretty much run a lot of numbers and birds are massively constrained by their take-off strategies. They leap with the legs and then flap with the wings, so they suffer a dual penalty – fisrt of all the legs are not that powerful so can’t do a great leap, and secondly the bird then has to lug around all the extra ‘take-off muscle’ in the loegs while flying. Pterosaurs (and indeed small bats, go look at vampires on the ground) however are driving off primarily with the forelimbs, the ones carrying all the huge and powerful flight muscles!. It makes quite a difference and more so at large scales.

    Quite what limits large bats I’m not sure is known (and I know Mike is working on this very issue) but birds may also be proportionally heavier than pterosaurs so for a given mass you can have a bigger pterosaur than a bird (a different but related issue of course to the highest possible mass). The lack of them today could well be due to extinction – we’ve lost a lot of big stuff in recent millennia.

    1. But doesn’t that just beg the question of what stops birds from adopting an arms-first takeoff strategy? Is it because they lack hands to push off with?

      1. Essentially, yes. Adopting a quadrupedal launch strategy basically requires that the animal already be adapted to a quadrupedal mode of travel on the ground. Birds are obligate bipeds derived from bipedal ancestors.

        That said, there is one context where birds can contact the surface with their wings effectively for launch, and that’s on the surface of water. The most powerful water launching birds (those that can do so in a single leap) are quadrupedal water launchers (includes some ducks, for example). Osprey are nearly forelimb-only launchers (basically reverse bipedal), and they are among the better water launchers, as well.

  2. Eh? I was driving in to work at that time this morning and Chris Packham was chatting about the avocet. Did I dream it?


  3. Do hollow bones play a part? Condors and some seabirds have very long wingspans, but clearly don’t tip the scales too deeply. What about penguins and other flightless birds? Where do they fit into the equation?

    1. Do hollow bones play a part?

      Yes, but not the way you’re thinking, I suspect. Both bird and pterosaurs have/ had hollow bones. As I’ve hinted elsewhere, it is plausible (I think – it’s not what I get paid to know about) that hollow bones are a primitive character for all members of the Archosaura (birds, other dinosaurs, pterosaurs, …. ) but were lost secondarily in some lineages.

  4. I’m just throwing this out there, but could it have to do less with mechanics and more with the ecosystem? Being big takes a lot of resources. If resources are harder to get (smaller prey, fewer plants, etc), wouldn’t we expect smaller body sizes? Of course, that just moves the question back. Why did land animals get smaller? Why is plant life less lush than it used to be?

    1. I was thinking of a somewhat related thought: maybe the lack of big birds is a secondary effect. Maybe something caused big-bird prey to go extinct, and the bird’s extinction followed.

      On it’s face it doesn’t make a lot of sense because we have lots of medium sized land and sea critters they could feed off of. However, AIUI most predators have a fairly narrow range of prey species. Maybe they could eat a wider variety of things, but AIUI, most of them don’t. So it probably wouldn’t take the elimination of more than a few prey species to cause the extinction of its predator bird.

  5. Why can’t the modern swift take off from the ground? And what is a modern swift? I found this:
    Healthy Common Swifts can take off from the ground easily, sometimes by pushing off with their long wings, but the ground must be smooth, so that the wings can flap freely. A Swift is not able to take off in long grass, for example.

    Is a modern swift a species (according to Wikipedia, there are over a 100 species of swifts)?

    If a swift finds itself on the ground, can it never take off again and is condemned to walking the rest of its days?

  6. Do very large birds simply provide more attractive targets for terrestrial hunters? I would imagine that the larger they are the more clumsy they would be on the ground and, thus, the more easily to be nommed on?

    1. That occurred to me as well, if you look at a swan taking off from a river, it takes a long time to get airborne. So one might imagine that bigger birds would be very vulnerable to predators. Also, of course it must take a hell of a lot of energy to take off, so that could be a factor too.

    2. This is my vote. There are many terrestrial hunters quite good at catching the current crop of birds when they try to take off. I expect the really big ones simply got et.

  7. Imagine a 30kg bat. That is an awful lot to feed with insects caught while flying. Bat have to be small to be manoeuvrable & be able to catch moths. Bats hang to roost – a very large bat could not do that – it would have to evolve a different way of roosting.

    Another question would be why do bats fly at night & why are more birds not active at night?

      1. I did not forget them – I deliberately did not mention them! They are quite different from the other bats, & I suppose (?) diverged a long way back. Why are they only 3.5 lbs then?

        Let’s face it, most animals are small. It makes sense that there is a pyramid with the base as numbers & the height as size, so very few enormous mammals but many small ones.

        A good book on size is Why Elephants have Big Ears by Chris Lavers.

        1. I thought that the Elephants have Big Ears because Noddy won’t pay the ransom. (You may have had to grow up in England to understand that)

  8. A related question – which possibly might have some bearing upon this one – is “why did some big birds lose flight?” The largest extant bird is, of course, the ostrich and I am wondering if the same evolutionary pressures that caused the ostrich ( plus others of course) to become flighless also explains why there are no big flying birds …

    Just a thought …..

      1. I don’t know (& I should have said that it was the ancestor of the ostrich that lost the ability to fly rather than the ostrich itself). My feeling is that they ( the ancestors) did have ability to fly but then lost it.

        Maybe they find going Ryanair easier ….

      2. They flew. Ostriches and other ratites are embedded in a big group of flying birds extending far back into the Cretaceous and a bit before.

    1. Ratite ancestors were probably similar to the extant tinamous. They fly but are essentially ground birds and are relatively small (up to 2 kg).

    2. Hmmm, selection pressure for large size can be due to selection for access to mates and for large #s of offspring. If they already nest on the ground and they do not migrate then that might set them up for less flying, and eventual no flying.
      Big flying birds tend have a very low reproductive rate, I suppose due to the massive energy constraint on producing large eggs and rearing chicks while still being able to fly. That is why many of the large bird species become so vulnerable when humans put pressure on them.

  9. Part of the answer may reside in the fact that the m. supracoracoideus – the muscle responsible for wing elevation – is too constrained in modern birds. This muscle is located ventrally (along the carena, under the m. pectoralis) and is connected to the the humerus by a long tendon running around the procoracoid bone. This morphology may restrict the maximal force that it can generate. Huge birds may have difficulties to flap their wings (that must be correspondingly heavier) fast enough to sustain flight.

    In ancient birds as in reptile, the homologs of the supracoracoideus were inserted more dorsally as the procoracoid was not as “distorted” as in modern birds.

    This would not explain why the bat are relatively small, still.

    Desnes Diev

    1. I think you have a confusion between sorts of ancient birds. All the really big ancient birds (ancient in time, i.e. pre-Holocene) were modern birds (modern in anatomy, i.e. members of crown group Aves). So your explanation of a change in anatomy doesn’t work, as it happened very early in avian history, considerably before the evolution of crown group Aves.

      1. Do you have an example of a large flying bird having a anatomically modern procoracoid? Can you refer me to a study on the evolution of the pectoral girdle in birds? Many thanks in advance.

        Desnes Diev

        1. All large flying birds have anatomically modern procoracoids, as they all lie within the clade Aves (or Neornithes if you prefer). Unless you consider “anatomically modern” to lie within some restricted clade within modern birds, there’s no avoiding this.

          By the way, the procoracoid isn’t a bone, at least not in birds; it’s a process on the coracoid. You’re really talking about the triosseal canal, I think.

          I don’t know of a review on the evolution of the pectoral girdle, but any such review would involve evolution prior to the crown group. The triosseal canal is present in several birds outside Aves, the earliest known being Protopteryx from the Liaoning deposits.

  10. Why are so many big warm blooded animals extinct overall in the last 10K years? Maybe the lack of really large flying birds is related to this issue and has nothing to do with constraints on flying size in the first place.

    1. It’s only the last 10K years in North America, extinctions there and elsewhere correlate with arrival of tool-using primates. A causal connection has been suggested.

  11. What about the fact that the air might have been a different density when pterosaurs were alive, due to differences in CO2 concentrations and temperature and oxygen levels? Might that not affect bouyancy and flight mechanics?

    Remember, some of our biggest animals occurred when oxygen levels reached 30%.

      1. Oops, missed Matthew’s comment, sorry.

        However, I would like to add that this thread would make a fantastic teaching tool for students – the variety of hypotheses for why there is lack of large flying birds today would be eye opening for them and a great example of how scientists think and do science. Thanks for everyone for their contributions to this really interesting subject.

  12. Did most large birds become extinct before man the hunter developed weapons which extended our kill range beyond our immediate vicinity?

    If not then they surely became extinct once such weapons were developed.

    Most if not all large birds would seem to be sitting ducks (no pun intended) when land-bound. Heck, I’d put out a carcass near cover but plainly visible from above and reap the rewards over and over.

  13. I’m going with people for 10 please.

    We don’t exactly have prey big enough either for large raptors? So a predator prey relationship may explain large birds of prey but lack of sufficiently big prey may limit the potential size of a bird.

  14. Dave Hone already covered most of the bases on launch and the effect that launch mode has on maximum size. I’ll just add in that one of the limiting mechanical factors is strength:weight ratio of the skeleton. Hollow, air-filled bones can be much larger in diameter than marrow-filled ones without becoming super heavy. Large diameter gives lots of strength, so air-filled skeletons can be stronger for a given weight of bone than marrow-filled ones. Birds and pterosaurs both had some marrow-filled bones, of course (need that to make blood), but much of their skeleton is/was air filled. As a result both birds and pterosaurs can achieve very high strength:weight ratios with regards to their skeletons.

    Bats, lacking air-filled skeletal systems (what we call “pneumaticity”) are constrained to a smaller size than birds or bats because a hypothetical giant bat would have either an insurmountably weak skeleton or an insurmountably heavy skeleton. Interestingly, though, there is no indication that any living bats are near the mechanical limits for the group (they can’t get huge, but they could, hypothetically, be larger than they are). As suggested by a few commenters already, this means that ecology is probably constraining them more than mechanics, at least for now.

    As for the utility of micro-blogging. Sure, we don’t know what keeps modern birds small. But there is a lot known about the mechanics of size and takeoff in flying animals, as seen in the Tw**ter stream. I presume some readers found that information interesting.

  15. Actually, to me, this is a perfect example of why I don’t like Twitter. When a few people mentioned takeoff strategies on Twitter, Dr. Coyne* relied on his prior knowledge to assume they were talking about dropping off of cliffs, rather than the quadrupedal launch style to which they were actually referring. Here in the comment section of this website, David Hone had enough space to at least mention the quadrupedal launch and explain briefly how it differs from the leaping takeoff of birds. Twitter’s just too constrained.

    *I know Dr. Coyne has mentioned in other posts that it’s okay to call him by his first name, but I can’t bring myself to use that level of familiarity. I guess I’m too old fashioned (even though I’m not that old).

    1. It’s Dr. Cobb actually, it’s a shared blogg so Matthew added his name under the title.

      Actually Jerry has mentioned that he _prefers_ first name basis. So I had to drop my cultural prejudices. And as can be seen, I now employ the same type of handle for all concerned here.

  16. The atmosphere question was previously addressed, but just as a quick note: in order for the atmosphere to have a major impact on flight in a large, vertebrate flyer, the total density would need to change by a rather substantial fraction. Air density has a larger effect on small flyers that rely on unsteady dynamics to generate high coefficients of lift. As it turns out, all evidence suggests that the Mesozoic atmosphere was only slightly different from the modern one, so the flight dynamics don’t change much (effectively the same as running the numbers at a very slightly different altitude).

  17. I tried to post a comment a moment ago about bat constraints, and it seems to have failed to post. A quick second try: bats are more constrained than birds or pterosaurs because they lack pneumatic (air filled) skeletons. This means that at large sizes, to maintain structural safety factors for flight, they would need a prohibitively heavy skeleton (or, to look at it another way, to have a skeleton that was light enough, it would need to be very weak).

    Interestingly enough, the total skeleton mass in a bird is not much less than that of a mammal of the same total body mass. The difference is that the diameter of the bird bones tends to be much greater, which makes them stronger for that weight. Air filled bones provide better Strength:Weight ratios.

    Interestingly enough, though, bats should be able to get larger than they are from a sheer mechanics perspective (not near the skeleton limits). A bat with a 3-4 meter wingspan is probably mechanical feasible. So ecology is probably limiting them, exactly as suggested by some commenters above.

  18. that micro-bl*gging site is not just for knowing what celebrities had for breakfast or for launching cyber lynch mobs.

    It is already a valuable science tool – at least within astronomy.

    I hear the latest rare relatively near supernova the other week got immediate and global observation, because Tw*tter was the de facto substitute for an administrative reaction. (Which there may be no set global channels for anyway, for all I know.)

    [Unfortunately it was the wrong type for neutrino observation, too weak generation, so I’m partly bummed anyway. But it was the right type for distance calibration, so astronomers if not physicists were sw**ning in the bl*gs.]

  19. I’m late to this, but I’ll just add: TetZoo is the one thing that is making me consider joining Twitter. It flippin’ rocks.

  20. What’s so great about being big? There seems to be an assumption that birds would be bigger if they could be, but I don’t see why that should be the case.
    Maybe they just don’t need to be bigger. Maybe the question should be why did previous flying animals get so big? Or even why have any big animals gotten so big? What’s in it for them?

      1. Yes, but why does that make bigger win? How does being a bigger bird help? I would guess faster was more useful.

    1. I imagine that the main advantage of being big is that you can eat a lot more and go for longer periods without dieing of starvation. If you are as small as a hummingbird, for instance, you only have minutes to find the next flower. Of course, with birds, that advantage is going to be offset by the higher energy costs of flying. Many larger birds such as pheasants are very reluctant to get off the ground.

      1. Another thing that occurs to me about being large is that you have more body mass in order to resist changes in temperature and wind chill and you can for instance have layers of fat to enhance that effect. So, if this is correct, one might expect birds in cold climates to be on average larger than those in warmer climates. Something the size of a sparrow could have a hard time in the Arctic.

  21. I’m speculating that pterosaurs could be giant flyers because they had membrane from feet to wingtip and between feet providing more wing surface area to glide and lift off the ground (like a kite). Birds use legs and thus are missing wing surface area relative to their body plan. Bats have high surface area wings but evolved into low calorie niches (fruit and insect diet) so their size is protein-constrained and they may be also be smaller in size because they’re mammals and face thermoregulation issues when large. Pterosaurs ate fish and could grow huge because of the reservoir of high-calorie, high-abundance food (think Alaskan brown bears and salmon). Are there fish-eating bats?

    1. Good thoughts all around Jimbo. Having a hind limb attachment for the wing might have increased effective wing area a bit for pterosaurs, though many of them likely had a narrow wing outboard of the elbow that turned to reach the ankle, which limited the effect to some extent. Possibly a more important factor regarding membrane wings is the higher maximum coefficient of lift they can achieve relative to a bird wing. Those factors still don’t completely close the size gap, but adding the difference in launch takes care of the rest (with room to spare).

      As an aside, only a few groups of pterosaurs likely ate fish. Many species had other diets, and the largest species of all seem to have lived inland and fed on small terrestrial animals.


    1. Great point. Flying feather replacement has to be energy intensive. Also, feathers must layer to make an airtight wing and being hollow can only get so long. I can’t imagine a 4 ft long wing feather that could take the stress of flying. An ostrich’s tail feather is in no way even close to a wing feather. A membrane wing solves all of this. To Dave’s point, imagine being a 5 ft tall eagle and losing several wing feathers. I’ll wager flight ability goes down considerably (and fitness with it) and how long to grow a replacement?

  22. Prof. CC – “They might as well have tw**ted what they had for breakfast!”

    Eggs, perhaps. This shows how even the largest mose ferocious birds, pterosaurs can be vulnerable to predation. This seems to create a selection ptressure to be smaller.

  23. I have another related question. Why there are not super-large and heavy flightless birds alive today or even in the past? Why no one reached dinosaur proportions and most of them became extinct before recent times? Are big flightless birds competitively inferio to mammals? And why some clades of birds became

    1. The Maori encountered several species of large, flightless birds (the moa)in New Zealand and hunted them to extinction. So large flightless birds are at a competitive disadvantage with respect to one species of mammal. Before the Maori arrived the only predator of the moa was the enormous Haast’s Eagle, now extinct.

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