lorikeet, Henderson reed warbler and the flightless Henderson crake. Three species of the family Columbidae – the Henderson ground dove, the Henderson imperial pigeon and the Henderson archaic pigeon – as well as the Henderson sandpiper, were formerly endemic to the island, but became extinct when the Polynesians arrived around 1000 CE. Of the fifteen non-endemic seabird species found, nine or more are believed to breed on the island. Breeding colonies of the globally endangered Henderson petrel formerly existed on Ducie but were wiped out by invasive rats by 1922. It is believed to now nest uniquely on Henderson island. Bones associated with prehistoric Polynesian settlement sites dating to somewhere between 500 and 800 years ago include those of the Polynesian storm petrel, Marquesan imperial pigeon, and Polynesian or Pacific imperial pigeon which are no longer found on the island, and two others – Christmas shearwater and red-footed booby – that still visit but no longer nest. It is hypothesized that the Polynesian settlers may have driven these bird species, along with six terrestrial snail species, to local extinction, and this loss of a ready and regular food supply may have contributed to the Polynesians' subsequent disappearance. The island has been identified by BirdLife International as an Important Bird Area for its endemic landbirds and breeding seabirds. Other animals The invertebrate species are largely unknown but a third of the island's known non-marine gastropods and insects are endemic. There are no native mammals but the Pacific rat, introduced by Polynesians 800 years ago, abounds. A skink (Emoia cyanura) and the green sea turtle have been identified, and an unidentified gecko has been reported. There are also crabs. Biological risk Land bird populations appear to be relatively stable but there is a high risk of introduction to the island of predators, disease vectors, and diseases by unauthorized landings of yachts. Introduction of the Eurasian black rat or the domestic cat would be likely to cause almost immediate extinction of the ground-dwelling Henderson crake and possibly other species. The endemic birds may have no immunity to the fatal avian pox which is transmitted by biting flies such as Hippoboscidae. Between July and November 2011 a partnership of the Pitcairn Islands Government and the Royal Society for the Protection of Birds implemented a poison baiting programme aimed at eradicating the Pacific rat. Mortality was massive but of the 50,000 to 100,000 population, 60