This is Dwarf Fortress. Critters that couldn't possibly exist, exist. And that's OK.
Take rocs, for example. IRL, birds can't fly above a weight of 40 lbs or so, because either their wing muscles get too heavy for their leg muscles to support, or their leg muscles are too heavy for their wing muscles to lift. (Pterosaurs overcame this limit by walking/launching using their wings, which meant their flight muscles could do double duty, but that's not consistent with the avian body plan.)
But giant birds are cool, so we have giant birds.
If you wonder how the creatures work, and other science facts, just repeat to yourself: "it's just a game. I should really just relax."
You contradict yourself there Bumber. Nothing inherantly about being a bird keeps our rocs from being able to do things the pterosaur way, not that the flight physics are exactly realistic as they presently stand anyway; given the lack of detail in the actual body plan for all we know that is how rocs take off.
As Dirst notes, I'm Button.
If rocs obeyed the laws of physics, the fact that rocs have feathered wings would keep them from doing it the pterosaur way. It would require the roc to walk on its wings, which would damage its feathers. Damaged feathers create a ton of drag. Un-feathered surfaces on feathered wings would also create a ton of drag.
Even positing a vertebrate flier optimized for taking off at a relatively immense weight, there is an upper bound beyond which the muscles required for takeoff are too heavy to lift themselves. I don't know exactly where that limit is, but I suspect it's no larger than a bull elephant.
Which is why it's a good thing that DF critters don't need to obey the laws of physics!