Neighbouring islands had different food: some had plenty of nuts but few insects, whereas others had more insects but few nuts. When mutation created birds with large bills the “nut” islands enabled them to survive, while the thin-billed birds there died out. BUT birds mutating with thin bills were able to survive on the “insect” islands, where the larger-billed died out. Where animals fit into the environment in this way is called Survival of the fittest.
