Before Human Colonization of New Zealand by the Maori 1,200

Before human colonization of New Zealand by the Maori 1,200 years ago,there were no land mammals and many species of flightless birds on the island.Why would there be more flightless birds on New Zealand than on nearby Australia,which had land mammals for millions of years?

A) Because only flightless birds colonized New Zealand.
B) Because birds evolved separately on New Zealand, and these birds had not yet gained the ability to fly.
C) Because the birds that colonized New Zealand adapted to many new environments.
D) Because the birds that could fly all left the island.
E) Because of competition with mammals.