Around 5,000 birds have been found dead in Esperance, Western Australia since mid-December, according to Nigel Higgs, spokesman for the state's Department of Environment and Conservation.
The birds were mostly nectar- and insect-eating species, although some seagulls also have been found, Higgs said in a telephone interview from his office in the Western Australia capital, Perth.
Pathologists at the Western Australia Department of Agriculture examined several of the carcasses, and have ruled out the virulent H5N1 bird flu virus and other infectious diseases.
"It may be an environmental toxin. It may be an agricultural or industrial toxin. We just can't be specific," Higgs said.
Further tests were being done on the dead birds, and Higgs said that it would be at least another week before pathologists have any more information on the mysterious deaths. Meanwhile, the reports of dead birds were waning, he said.
Meanwhile, agricultural officials ordered a nationwide inspection of poultry farms Friday after hundreds of chickens died in southern Japan this week in a suspected bird flu outbreak.
The government order followed the deaths of 750 chickens Wednesday and Thursday at a farm in the town of Kiyotake in Miyazaki prefecture (state), an Agricultural Ministry official said on condition of anonymity, citing protocol.
The results of a preliminary bird flu test came back positive early Friday evening, said prefectural official Masao Tanaka, but he cautioned that the final results would not be known until at least early Saturday.
Bird flu is generally not harmful to humans, but the H5N1 strain of the virus has claimed at least 157 lives worldwide since it began ravaging Asian poultry farms in late 2003, according to the World Health Organization.
Around 1,600 more chickens had died by early Friday evening, bringing the total to about 2,400, Tanaka said.
The government has set up a task force and ordered 20 poultry farms within a 6.2-mile radius of the affected operator to halt shipments of eggs and chickens for the time being, said Toru Inoue, another prefectural official.