Yo the catch 22 of birds sounds like an incredibly interesting lecture please elaborate :0
Okay so the thing is, feathers didn’t evolve for flight. They were insulation and display first, and they’ve been around since before the dinosaurs: some of the most recent research (1) suggests they go back to or before the common ancestors of pterosaurs and dinosaurs, aka the mid-Triassic or earlier.
In addition to THAT, basically all dinosaurs came from bipeds(2): the big guys like sauropods and stegosaurus were secondarily quadrupedal. As such, when birds started to evolve towards flight, they were already both covered in feathers and bipedal.
There’s a debate (3) on whether bird flight evolved “trees down”, where they evolved gliding first, jumping out of trees and using their long arm and leg feathers to slow the fall, or “ground up”, where they evolved FLAPPING first, using long feathers on their outstretched arms while running to help keep them balanced and help them jump higher while hunting or escaping predators.
Personally, I prefer the synthesis hypothesis which suggests a combination of strategies including using wings to slow their fall, run, and help gain height when getting into trees/cliffs, but that’s not relevant right now.
What’s important is the fact that by the time birds developed flight, they’d locked themselves into having hyperspecialized limbs: legs ONLY for running and jumping, arms ONLY for flying and flapping.
Since they were small, this wasn’t a problem! But it doesn’t scale super well, and to see why, let’s take a brief tangent into pterosaurs.
Pterosaurs are the biggest fliers that have ever existed, with some species having an estimated mass of up to 550lb. Despite this, even the largest individuals would have been able to take off without an assist, for one reason: they got off the ground with a “catapult” launch(4)(5).
Pterosaurs were quadrupeds, but that’s not their biggest asset. They used their wings for flying, as you’d expect; however, they also used them on the ground with a gait that functioned similarly to a gorilla or a vampire bat, “front wheel drive” as it were. With a catapult launch, they “jumped” off the ground primarily with their wings, resulting in the same primary muscles being used across all forms of locomotion.
This means that, if they needed to get bigger, pterosaurs could just keep pumping more energy into making the wings bigger as well, with no impact on their ability to fly or take off: it’s hypothesized that the largest pterosaurs were limited more by their hearts’ ability to pump blood to their extremities than they were by the ability to fly.
By comparison, in order to get bigger birds have to both add more leg AND add more wing. More weight needs bigger wings to lift it, but bigger wings need bigger legs to jump high enough for the wings to flap, but bigger legs need bigger wings to carry them during flight, and so on.
There comes a point where the legs can’t jump high enough to give the wings room to flap, and then they’re stuck. A lot of modern larger birds are partially reliant on gravity/wind assists or take off from the water because they can’t easily get high enough on their own, and these birds are all like, under 40lb. Even with environmental assists, balancing two sets of limbs to maintain the ability to both walk AND fly means that birds have a size limit: the largest flight-capable bird (6) that’s ever existed had a mass of up to 160lb, and probably wouldn’t have been able to get off the ground at all without a strong headwind or a cliff.
Essentially, in order to get bigger birds have to both add more leg AND add more wing but there’s a limit on how much they can add of each before they just can’t get off the ground, and that’s the catch-22 of birds!
1. https://www.nature.com/articles/s41559-018-0728-7
2. https://en.wikipedia.org/wiki/Kongonaphon
3. https://en.wikipedia.org/wiki/Origin_of_avian_flight
4. https://youtu.be/9TKgupAZVzE
5. https://youtu.be/ALziqtuLxBQ
6. https://en.wikipedia.org/wiki/Argentavis
