Orange-bellied parrots, Neophema chrysogaster, have been hit with a stomach virus that causes them to lose their feathers and weakens their immune systems, ScientificAmerican.com reported.
Under Australia's Environment Protection and Biodiversity Conservation Act, the orange-bellied parrot was listed as critically endangered in 2006.
The species, once common in Australia, has been in decline for the past 100 years as its coastal salt marsh habitat was destroyed for agricultural purposes.
About 160 to 170 orange-bellied parrots are in three captive breeding programs, two on the Australian mainland and one on the island of Tasmania. Only about 50 of the birds exist in the wild.
The nature of the virus has not been established, although some scientists theorize it could be avian influenza or proventricular dilatation disease, which hampers food digestion.
Because the captive parrots live in close proximity to one another, "infections just find it easier to spread from one bird to another," Shane Radial, a veterinary professor at Charles Sturt University, said.