Why can't Rheas fly?

Answer 1

Although rheas are birds, they are one of the very few birds that can't actually fly at all. They are members of a group of birds called ratites. Other ratites include the Southern cassowary of Australia and New Guinea, the kiwi from New Zealand, the ostrich from Africa and the emu from Australia.

Ratites have wings but the bones in their chests do not have the capacity for flight muscles, which is what a bird needs to fly. Although the rhea does not actually weigh as much as it appears (20-27kg), its wings are too small by comparison to lift it.