For 10 million years after the K–Pg extinction event mammals remained small. This ecological vacuum gave a window of opportunity for the evolution of large herbivores. Birds that had flown to occupy the islands of Madagascar (from Africa), Mauritius (from India) and New Zealand (from Australia) upsized, as did a few terrestrial birds on continental landmasses.
Putting on weight put getting off the ground out of reach. Wings withered. So evolved the large flightless birds: the ostrich in Africa, elephant birds in Madagascar, the dodo in Mauritius, the emu and cassowary in Australia and New Guinea, the moa and kiwi in New Zealand, and the rhea in South America.