Have a read of the wikipedia entry for Garuda, the mythical gigantic bird-gods of Asian legend. Check out this paragraph:
Garudas are the great golden-winged Peng birds. They also have the ability to grow large or small, and to appear and disappear at will. Their wingspan is 330 yojanas (one yojana being 8 miles long). With one flap of its wings, a Peng bird dries up the waters of the sea so that it can gobble up all the exposed dragons. With another flap of its wings, it can level the mountains by moving them into the ocean.
A bird with a wingspan of 2640 miles. I can only speculate on how big that would make a feather, but think about what kind of societies might exist buried down beneath the plumage in what would presumably be near-darkness. Underdark creatures burrowing through the skin. Cities built into the hollow spaces within the feathers (living on the brink of potential apocalypse when a feather comes loose or the bird gets too vigorous with its preening). Blood farmed (mined?) for food or other purposes. Constant efforts to overcome radical shifts in the direction of gravity as the bird alternates between flying and perching upright. Legends of the great open space above, which can only be seen by climbing far, far upwards beyond anybody's wildest imagining...
Maybe I ought to stop reading wikipedia entries.