After the ash and lava slopes were covered with vegetation and an insect population had been established, the island began to be colonized by birds. The first birds to arrive were the Australian golden whistlers, which were blown across the ocean from Australia. Originally a fairly unspecialized bird it had, during the Quaternary, begun to show some differentiation, with distinct beak shapes developing on the islands around the Australian coast. However, it was only on the Pacaus Archipelago, where all the ecological niches were thrown open to them, that the whistlers really developed spectacularly, producing both insectivorous and seed-eating as well as highly predatory forms. They became the Pacauan whistlers.
The only natural enemies faced by the Pacauan whistlers are the snakes, which have been rafted to Pacaus from Australia or the other islands in that corner of the Pacific at one time or another.
There are only a few mammals living in the archipelago.