eli5: What limits how big a bird can be and still be capable of flight?
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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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