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?
Lots of great answers already, but one interesting thing that seems to be like it would keep more mammals from evolving to fly:
extra weight of the young during gestation.
Egg-laying birds get to breed but don’t carry around that extra weight for nearly as long or as heavy. That’s got to be a huge evolutionary advantage, right?
Ducks seem to go and nest to lay anyways. Stick around one spot for up to 15 days shitting out eggs, then sit for another 30 days waiting for them all to hatch. Sit, rotate eggs, sit, barely leave, repeat. Then teach them to swim/fly etc before they do the whole flying away part