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I wonder whether the birds or other flying animals heavier than air could evolve without trees of other protruding objects?

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    $\begingroup$ What makes you think trees are needed to evolve flight? $\endgroup$ – John Aug 1 at 22:37
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    $\begingroup$ How many trees are on Antartica? $\endgroup$ – Cyn Aug 2 at 0:17
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    $\begingroup$ @Cyn: How many flying birds evolved on Antarctica? (Disregarding those times when plate tectonics had it in a warmer location, so there were trees.) $\endgroup$ – jamesqf Aug 2 at 4:08
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    $\begingroup$ @jamesqf The Antarctic Petrel, and the South Polar Skua. And, of course, there are plenty of other locations with no naturally growing trees, such as the Falkland Island (home of the Falkland Steamer Duck), although with extensive windbreaks it is sometimes possible to cultivate trees. $\endgroup$ – Chronocidal Aug 2 at 10:26
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    $\begingroup$ Many sea birds in don't use trees for nesting, though they do use ledges on cliffs etc. In fact sea birds with webbed feet are pretty much incapable of landing on anything that resembles a tree, and certainly incapable of building a nest in one - many of them don't build nests at all. $\endgroup$ – alephzero Aug 2 at 11:08
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Trees were not involved with the evolution of flight in birds.

Birds did not evolve flight from gliders but likely from ground running predatory jumpers, birds and maniraptoran dinosaurs are about the most poorly designed climbers you could imagine there is zero support for tree climbing in early birds or their ancestors. So yes bird flight can and did evolve without the use of trees.

I should note birds are the only group of flying vertebrate this is true for, pterosaurs and bats did evolve from climbers.

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    $\begingroup$ One should emphasize that opinions on this subject vary quite a lot. $\endgroup$ – AlexP Aug 2 at 8:45
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    $\begingroup$ @AlexP opinions, perhaps, but not reasoned facts. Birds & dinosaurs are on the same branch, and no evidence I'm aware of that either ever developed climbing capabilities. $\endgroup$ – Carl Witthoft Aug 2 at 12:31
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    $\begingroup$ @jamesqf I'm an experienced birder. To the best of my knowledge, these birds developed their trunk-climbing abilities long after their ancestors took tothe air. $\endgroup$ – Carl Witthoft Aug 2 at 16:59
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    $\begingroup$ There is a theory that wing assisted incline running evolved first, and then flight grew out of that. Youtube has some good videos of WAIR videos, like this one: youtube.com/watch?v=b1dekSaGhlc and this one youtube.com/watch?v=JMuzlEQz3uo. $\endgroup$ – Garrett Motzner Aug 2 at 17:36
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    $\begingroup$ That is one way to look at it: wings created for flight are useful for incline running. But the theory proposes the opposite: wings created for WAIR are useful for flight, and then the usefulness of flight eventually becomes the primary purpose, with WAIR being a latent trait. This seems plausible because WAIR needs simpler structures than fully developed wings to provide an evolutionary advantage. Eventually those structure improve, adding to the advantage of WAIR with each step, eventually leading to flight. $\endgroup$ – Garrett Motzner Aug 2 at 18:52
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The first tree is probably 385 million years old. The first winged insect is probably 400 million years old.

These numbers are approximate, but our understanding of geology and palaeontology should mean these are accurate enough. Insects seem to predate trees by 15 million years. That is a lot of time - about the same distance in time between now and the release of the last book in the Game of Throne series!

To be honest, though... About 430 million years ago there were fungi whose fruiting bodies could reach up to 8m in height. They could have served the same role as trees for insects. And they predate insects by about 30 million years, or about the timespan between the roman empire and the release of Half Life 3.

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    $\begingroup$ Possibly, there were high grasses as well. $\endgroup$ – Anixx Aug 1 at 21:57
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    $\begingroup$ The aerodynamics of insects - especially the smaller ones - is much different than that of birds, though. $\endgroup$ – jamesqf Aug 2 at 4:10
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    $\begingroup$ @jamesqf yeah but OP said "birds or other flying animals heavier than air could" $\endgroup$ – Renan Aug 2 at 11:59
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    $\begingroup$ I see what you did there... $\endgroup$ – TzeraFNX Aug 2 at 13:28
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    $\begingroup$ @barbecue Considering that that would exclude cliffs, mountains/hills, moderately-sized boulders, and large waves (in case a small dolphin-like creature developed fins akin to a flying fish, and then learned to extend airtime by flapping), I think "protruding objects" is a bit too vague. $\endgroup$ – Chronocidal Aug 4 at 12:24
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Probably.

The most accurate answer to a question like this is always going to be “We don’t know.” Evolution is an incredibly complex and fundamentally random process so there are no definitive answers here. But, that said, I think there’s good reason to believe flight probably would have evolved even without trees or other protrusions.

Flight is thought to have evolved 4 separate times on Earth. In insects, pterosaurs, bats, and birds. This suggests that flight isn’t extraordinarily difficult to evolve and serves as a useful adaptation in a variety of environments for a variety of organisms.

While trees certainly play a large part in the lives of plenty of birds, many birds thrive in treeless environments. Waterfowl like ducks spend their time swimming and generally build their nests on the ground. Seabirds like cormorants spend their lives fishing and often nest in colonies on the ground. There are also ground birds such as quails that nest and feed on the ground often in treeless areas. In all of these cases, some of the birds in these habitats have lost their ability to fly which implies that in the absence of trees flight isn't perhaps as critical to their survival. But most of these bird species living in treeless environments have retained their ability to fly which implies that it remains a useful ability for these organisms to find food, escape predators, or migrate.

Birds certainly tend to take advantage of trees when they are available but their success in treeless environments tells us that the safety of trees is not the only advantage of flight and suggests that there is ample reason for flight to evolve even in the absence of trees.

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    $\begingroup$ You can also add Pteromys volans and Exocoetidae, while not fully developed fly, a step towards it, definitely. $\endgroup$ – Anixx Aug 1 at 22:45
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    $\begingroup$ If you count gliders as parallel evolutions of flight, then flying toads Rhacophorus nigropalmatus, flying snakes Chrysopelea and flying geckos Ptychozoon kuhli should not be left out. Probably there end up being dozens of independent evolutions of flight. $\endgroup$ – Oxy Aug 2 at 9:44
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    $\begingroup$ I think you forgot humans. Without wood as a building material, human flight would have evolved much differently. $\endgroup$ – emory Aug 3 at 0:38
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Well, flying fish evolved without trees or other protruding objects to help lift them out of the water, so I'm going to say "yes". And the initial motivations would probably be the same: to better avoid predators.

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    $\begingroup$ Flying fish don't use powered flight, though. They glide. Aerodynamics is not that much different from hydrodynamics, but both are greatly different from running around on land. $\endgroup$ – jamesqf Aug 2 at 4:10
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    $\begingroup$ Non-flying birds also evolved. The ostrich and emu, for example do have trees around themselves but there are (usually) little to no trees around penguins. The roadrunner birds tend to live in the desert. And occasionally run on actual roads and be chased by a coyote. At any rate, they can fly but prefer running. Then we have ordinary chickens which are not completely flightless but almost never engage in it. Birds are not only defined by flying. $\endgroup$ – VLAZ Aug 2 at 10:46
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    $\begingroup$ @jamesqf Give 'em another hundred million years with no competition from seabirds. $\endgroup$ – Logan R. Kearsley Aug 2 at 15:29
  • $\begingroup$ @VLAZ: But all those non-flying, or seldom flying, birds evolved from ancestors that flew. $\endgroup$ – jamesqf Aug 2 at 16:53
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    $\begingroup$ @jamesqf Sure there is. While the precise sequence of the evolutionary path of bird flight is still debated, the cursorial origin, with proto-wings developing as balance assistants for high-speed running, then to control surfaces, then allowing short hopping flight, is a well-established theory in that space. For insects, it's even easier, starting with uncontrolled jumping--which is still seen without the addition of gliding or flight control in, e.g., fleas. $\endgroup$ – Logan R. Kearsley Aug 2 at 17:03
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While I agree with the existing answers, what it comes down to is the question of what benefit does a creature get that makes the adaptation of flight worth it?

There are two basic benefits that I can see that a flying animal may have over a land based animal; protection and ambush.

The first is obvious; if you can take flight, you can scan a larger area for threats, but also you can escape those threats if you're on the ground via a vector your attacker probably can't follow. As for the second, well if there aren't a lot of other fliers out there yet, perhaps your prey doesn't know to look up from time to time and won't see you coming.

Of course, this makes the most sense if the energy tradeoff is small, therefore it follows that the first flying creatures would be as small as is practicable, taking advantage of the square cube law. It's little wonder therefore that the first flying creatures were small insects.

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    $\begingroup$ Insects have to move their wings at very high rate when they fly (unlike birds and flying mammals). I have no idea of how worm-like creature could develop fast-moving wings. $\endgroup$ – Anixx Aug 2 at 9:11
  • $\begingroup$ lets not forget that hummingbirds exist. $\endgroup$ – candied_orange Aug 2 at 9:58
  • $\begingroup$ @Anixx: Not all flying insects move their wings at high speeds. Butterflies, for instance. Which coincidentally spend a large part of their life as worm-like creatures :-) $\endgroup$ – jamesqf Aug 4 at 17:40
  • $\begingroup$ @jamesqf butterflies have huge wing area to their body ratio compared to birds. $\endgroup$ – Anixx Aug 4 at 19:16
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    $\begingroup$ @Anixx: Yes, it's a tradeoff between area and speed. You see much the same tradeoff in birds, where you have a range from very fast flappers like hummingbirds to soaring birds like hawks & albatrosses that can fly for long periods without flapping. $\endgroup$ – jamesqf Aug 6 at 3:33
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Since plants need light, and the tallest plant gets the best light, then if there were no trees there would inevitably be very tall plants that aren’t technically trees. However, they would function much like trees for ecological purposes, and animals would still climb them, jump between them and fall off them, so the evolution of flying animals of any kind would be unchanged. If you’re going to arbitrarily change the way evolution works to rule out tall plants, then you can equally arbitrarily rule birds in our out, depending on what you want — you’ve already given up on evolutionary plausibility.

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