Like, I’m aware of there being exceptions like Penguins, Ostriches, and Bats. But in general, why is there such a distinct land/air split between mammals and birds? Why don’t mammals share the ground with ecosystems of plant- and meat-eating walking birds? Why didn’t we get birds that evolved to slither like snakes, or tunnel like rodents? Why isn’t it (land+sky) all just mammals, where we’d have parrot- and vulture-like bats that don’t lay eggs? If we started the simulation again, might things like this evolve?
Birds already had a sky advantage and mammals had a land advantage when the dinosaurs went extinct. It takes millions of years to develop things like feathers, why compete with animals that are far more specialized than you for the task at hand.
So I think it all comes down to competition, evolution fills niches that are open, if it’s already crowded then naturally it’s more beneficial to expand in empty niches.
Ah, that would make a lot of sense