DiseaseDisease

Disease

A serious threat to all island birds is the introduction of new disease. Island birds have often evolved in the absence of diseases common in continental bird faunas and the introduction of such diseases to island birds can be disastrous. For example the introduction of avian malaria to Hawaii which caused the extinction of almost the entire endemic bird fauna from below 600 m altitude, and was probably the main cause of the total extinction of several bird species (Hay 1986). The range of many surviving species was severely reduced and fragmented which in turn markedly increased their chances of extinction. Avian malaria arrived with the accidental introduction of a new species of mosquito (Hay 1986).

Once established, a new disease may persist on an island in feral populations of continental bird species which have resistance to the disease. The Black Francolin Francolinus francolinus, introduced to Guam, may have provided a reservoir of disease-carrying hosts accentuating the impact of disease on the island's endemics (Hay 1986). McNamara (1989), summarising the conclusions from a workshop on the management of Australian and New Zealand islands, recommended that the eradication of introduced species from islands be given a high priority.

Psittacine Circoviral Disease (PCD) was confirmed in wild and captive populations of Green Parrots in 1995 (Stevenson et al. 1995). The disease is prevalent in Crimson Rosellas on Norfolk Island (Hermes et al. 1986, Hicks and Preece 1991). An outbreak of PCD caused high mortality among Crimson Rosellas in 1976 and one apparently diseased Green Parrot was seen in 1977 (Forshaw 1981). In 1995 there was an outbreak of PCD in Green Parrots and Crimson Rosellas and two captive female Green Parrots died (Yorkston 1995). The disease is now thought to be widespread in the wild Green Parrot population. With the entire sub-species, captive and wild, located only on Norfolk Island, it is extremely vulnerable to a disease epidemic. Recent studies by Taronga Zoo suggests that all NI Green Parrots are carriers of the virus but appear to remain unaffected until subjected to stress. Green Parrots may be particularly vulnerable to disease due to the population bottlenecks the taxon has suffered in the past which may have reduced the genetic variability in the population. PCD is a nationally listed as a Key Threatening Process (EPBC 1999), and in June 2002 work commenced on a Threat Abatement Plan. The Threat Abatement Plan is likely to make recommendations which will affect the Norfolk Island Green Parrot and these will have to be considered by the Recovery Team. Captive breeding for release may exacerbate the spread of PCD (PCD Nomination; EPBC website). A vaccine is now available which induces immunity in vaccinated birds and might assist management of the disease at least in captive Norfolk Island Green Parrots.