As a relatively small and isolated oceanic island, Norfolk has few land birds but a high degree of endemicity among them. Many of the endemic species and subspecies have become extinct as a result of massive clearance of the island's native vegetation of subtropical rainforest for agriculture, hunting and persecution as agricultural pests. The birds have also suffered from the introduction of <answer>mammals</answer> such as rats, cats, pigs and goats, as well as from introduced competitors such as common blackbirds and crimson rosellas.
What non-bird animals caused the bird populations of Norfolk Island to suffer?
mammals