The Papuan kingfishers living in the mainland of New Guinea show little variation in bill size, tail feather length, and structure. The isolated population of Papuan kingfishers living on nearby islands differ from each other and from the mainland kingfishers.
Which statement would be the most direct reason that island kingfishers have greater variation as compared to the mainland kingfishers?
It is a result of migration from one island to another.
It is a result of adaptation to food and climate of the island.
It is a result of frequent climatic changes experienced on the island.
It is a result of competition between the island and mainland kingfishers.