An avian expert says the critically endangered western ground parrot could become the first bird in at least 200 years to become extinct in Western Australia.

Already critically endangered, the parrot's south-coast habitat was decimated by last year's catastrophic Esperance bushfires.

While two parrots were rescued and brought into Perth Zoo's captive breeding program, both birds died in captivity.

A post mortem by zoo staff confirmed the animals had died of aspergillosis, a respiratory disease common in parrots in captivity.

Birdlife WA chairman Mike Bamford said it was a particularly concerning development.

"We could be looking at extinction," Dr Bamford said.

"As everyone knows, the fires burned out 80-90 per cent of the areas the western ground parrot was hanging on."

He said the pre-bushfire wild population of the bird, believed to be fewer than 100, could have been reduced to as few as 30 or 40 by the devastation.

The western ground parrot is unique to Western Australia.

"If it goes extinct, it would be the first West Australian bird species to go extinct in European times," Dr Bamford said.

"We've lost a couple of subspecies, but not a full species. That's a horrible thing to be looking at."

Senior Perth Zoo veterinarian Simone Vitale (left) and vet nurse Jess Rice check the health of an endangered western ground parrot. ( ABC News: Pamela Medlen )

Zoo staff devastated by loss of parrots

A Perth Zoo spokeswoman said aspergillosis in parrots could be driven by a number of complex factors.

"The development of aspergillosis occurred despite the strictest protocols, 24-hour monitoring of the birds, and care from a team who have more than 125 years combined experience caring for and breeding an extensive variety of native and exotic parrots," she said.

"Staff are collecting information to determine any predisposing factors which may have triggered the infection in these birds.

"However, it may never be known."

The spokeswoman said older or younger birds, or those with pre-existing diseases or injuries, could be more susceptible to the condition.

The deceased birds were still under strict quarantine, and there is not believed to be any risk to the zoo's remaining five parrots.

The spokeswoman said the staff were devastated by the loss of the birds.

"To say our team feel the pressure is an understatement. It's been a hard few weeks," she said.

"It is not currently breeding season, but we will continue to pair the [remaining] birds, and care for them, in the hope of some having mating and breeding success."

The success or failure of the latest breeding efforts will not be fully known until spring.