Last updated at 22:00 10 January 2007

It could be the plot of a horror film, but in two towns on opposite sides of the world the mysterious phenomenon of thousands of dead birds dropping out of the sky is all too real.

Officials are baffled by the unexplained deaths which have affected Australia and the U.S.

Three weeks ago thousands of crows, pigeons, wattles and honeyeaters fell out of the sky in Esperance, Western Australia.

Then last week dozens of grackles, sparrows and pigeons dropped dead on two streets in Austin, Texas.

As birds continue to die in Esperance and the town's dawn chorus remains eerily silent, vets in both countries have been unable to establish a cause of death - despite carrying out a large number of autopsies on the birds.

Wildlife officials from Western Australia's Department of Environment and Conservation said they were baffled by the "catastrophic event" but emphasised the deaths had nothing to do with a severe storm which recently struck the area, as the birds had started dying before then. District nature conservation coordinator Mike Fitzgerald said: "It's very substantial.

"We estimate several thousand birds are dead, although we don't have a clear number because of the large areas of bushland."

Birds Australia, the country's largest bird conservation group, said it had not heard of a similar occurrence.

"You'd have to call it a most unusual event and one that we'd all have to be concerned about," said chief executive Graeme Hamilton.

Dr Fiona Sunderman, chief veterinary officer of the Department of Agriculture and Food, suspects the cause of death is some form of toxic poisoning.

Esperance resident Michelle Crisp, who normally sees hundreds of birds roosting in her garden, counted 80 dead ones in one day.

"It went to the point where we had nothing, not a single bird," she said. "It was like a moonscape - just horrible."

In Texas, officials are also working on the toxic poisoning theory. Adolfo Valadez, medical director for Austin and Travis County Health and Human Services, said it might be weeks before any conclusive results were known.

Such was the concern that the birds suffered deliberate toxic poisoning that several streets were closed in Austin while police and fire crews checked the area for any substance that might be of harm to humans.

"This was a precautionary measure. We certainly take these kinds of thing seriously, especially following 9/11," said Mr Valadez. "It may be a matter of time before we know what happened and why it happened. There is no threat to public health."

Federal officials in Washington said they were monitoring the situation, but a spokesman for the Department of Homeland Security said: "There is no credible intelligence to suggest an imminent threat to the homeland or Austin at this time."