What links the cassowary emu and kiwi?

The cassowary, emu and kiwi are linked because they are all Flightless Birds. Specifically, they are members of a group of birds called ratites. Other ratites include the ostrich from Africa and the rhea from South America. 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.