eli5: What limits how big a bird can be and still be capable of flight?
In: 12
The major limiting factor in the size of birds is the time it takes to regrow feathers.
Birds periodically shed their old and damaged feathers and regrow new ones in a process called “molting”. While a bird’s wing feathers are molting it’s ability to fly is severely reduced.
Small birds with small feathers can molt very quickly, so molting is not a very difficult challenge for small birds.
Larger birds with larger feathers may take weeks or months to finish molting. As larger and larger birds approach the upper limit of bird size, molting becomes a debilitating handicap that lasts for many months. The bird would be less to able to gather food and very vulnerable to predators.
In the age of dinosaurs, there were a group flying reptiles called pterosaurs that had wings made not of feather but of skin stretched over an elongated finger bone.
Pterosaurs were therefore not limited in size by molting, and some pterosaurs evolved to be as large as giraffes.
The wingspan of the largest known birds, Argentavis, was 6 meters.
The wingspan of the largest known pterosaurs, Quetzalcoatlus, was 12 meters.
The density of the atmosphere.
A larger planet with a higher gravitational pull will, counter intuitively, be able to evolve far larger avians than Earth.
As the gravitational pull is higher, the atmosphere is more dense, and therefore a heavier body can create enough lift to fly.
In our own Earth’s history, when there was more denser oxygen than in our current nitrogen rich atmosphere, [giant flying insects](https://entomology.unl.edu/scilit/largest-extinct-insect#:~:text=The%20largest%20insect%20ever%20know,about%20275%20million%20years%20ago.) existed due to the denser atmosphere, as well as more oxygen so they could breathe through their skin
Simply put, the weight of an animal increased with its dimensions^3 and the muscle power of an animal increased with its dimensions^2 . So if the size of a bird doubles, its weight increase 8x but its muscle power can only increase 4x. If you triple the size, the weight increases 27x, muscle power increases 9x.
After a certain size, the wingspan required to lift the weight of the bird is just way too big for the possible muscles on that bird. Here’s [the wingspan required for a human to fly](https://qph.cf2.quoracdn.net/main-qimg-8f3bebfc0974884474914ea055ddc327) and while we may have the muscles to do a few push-ups (lifting half our weight off the floor), we don’t even have enough surface on our chest bones to anchor the bulk of muscles we’d need to flap wings of that size.
The heavier the bird, the bigger its wings have to be to keep it aloft. Bigger muscles are needed as well. At some point the bird is so large that the required wingspan is MASSIVE. When a species hits the point at which their wings will no longer hold them aloft, they become flightless and generally evolve toward smaller or more ornamental wings.
On earth, a big limiting factor of bird size is that they have to launch off the ground with their hind limbs, but fly with their forelimbs. Pterosaurs were able to launch off the ground by essentially vaulting off the ground with all four limbs, allowing them to use their powerful flight muscles to help get airborne. Birds can’t do this, so they have to carry powerful leg muscles as well as wing muscles, which weigh more. Because of how weight and strength scale, this makes them hit a maximum size earlier than pterosaurs.
Here’s an article about it
https://www.hopkinsmedicine.org/news/media/releases/four_three_two_one____pterosaurs_have_lift_off
As for what the actual largest flying bird was, there are two top contenders. Pelagornithids were soaring, albatross-like seabirds with weird bony “teeth” in their bills. They probably had the longest wings, a bit over 20 feet, but were a bit lighter than our other contender. The heaviest bird was likely _Argentavis_, which was essentially a huge condor. It seems to have had a body weight somewhere around 160 lbs and wingspan a bit under 20 feet.
For comparison, pterosaurs seem to have maxed out with a wingspan in the low 30 feet range and a weight of around 500lbs.
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