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?
Different medium means vastly different things with be chosen for. Same with water based animals. It comes down to base physics of how to live and what’s advantageous evolutionary. The obvious one for birds is they need to be light weight to fly. That’s extremely limiting. Land based has a ton of options. Water based needs a ton of fat for insulation, can be heavier because of buoyancy.