The degradation of native forest on Kaua'i means a loss of habitat for the birds that threatens its continued existence on the island. The clearing of forest for timber, agriculture and urban development contributes to this, as does browsing by introduced species such as goats, while the spread of exotic plants is causing further habitat degradation (3) (4). Although the anianiau is relatively tolerant of habitat disturbance, it is more common in undisturbed native forest (2) (3) (5).
Another major threat to the anianiau is the introduced southern house mosquito, Culex quinquefasciatus. This mosquito is a vector for diseases that affect the anianiau, particularly avian pox and avian malaria. As a result, the anianiau is now largely confined to upland forests where there are fewer mosquitoes (4) (7). However, the increase in temperature expected as a result of climate change is likely to cause a corresponding increase in the range of the mosquito on the island, spreading these diseases to higher elevations (3) (4).
Other introduced species also have negative effects on the birds. Feral pigs exacerbate the spread of disease by creating wallows in which mosquitoes can breed, and also destroy understorey plants as well as spreading the seeds of invasive plants (2) (3) (4). Introduced predators such as cats, barn owls and rats are another very real threat to the survival of the anianiau (2) (3) (4) (5), and introduced birds can serve as reservoirs of disease (2) (4).
The anianiau also suffers from competition for food resources with introduced wasps and ants, and particularly with the Japanese white-eye (Zosterops japonicus) (2) (3) (5). In other native Hawaiian birds, this competition causes lower body mass that makes juveniles less able to resist disease. The lack of nutrition in early life also causes these birds to have stunted bill growth, which impairs their foraging ability (8).
Its restricted distribution makes the birds more vulnerable to any catastrophic events, such as hurricanes (3) (4). However, since its drastic reduction after the destruction of forest areas by Hurricane Iniki in 1992, the anianiau population seems to be on the increase (2), and this species is currently thought to be stable in its remaining habitat (2) (4) (5).