Thousands of birds have fallen from the skies over Esperance, Australia and no one knows why.

Is it an illness, toxins or a natural phenomenon? A string of autopsies in Perth have shed no light on the mystery.

All the residents of flood-devastated Esperance know is that their "dawn chorus" of singing birds is missing.

The main casualties are wattle birds, yellow-throated miners, new holland honeyeaters and singing honeyeaters, although some dead crows, hawks and pigeons have also been found.

Wildlife officers are baffled by the "catastrophic" event, which the Department of Environment and Conservation said began well before last week's freak storm.

On Monday, Esperance, 450 miles southeast of Perth, was declared a natural disaster zone.

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District nature conservation co-ordinator Mike Fitzgerald said the first reports of birds dropping dead in people's yards came in three weeks ago. More than 500 deaths had since been notified. But the calls stopped suddenly last week, reportedly because no birds were left.

"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," Fitzgerald said.

Birds Australia, the nation's main bird conservation group, said it had not heard of a similar occurrence. "Not on that scale, and all at the same time, and also the fact that it's several different species," chief executive Graeme Hamilton said. "You'd have to call that a most unusual event and one that we'd all have to be concerned about."

He expected birds would return to the area once the problem — natural or man-made phenomenon — was fixed but said it was vital the cause was identified.

The Department of Agriculture and Food, which conducted the autopsies, has almost ruled out an infectious process.

Acting chief veterinary officer Fiona Sunderman said toxins were the most likely cause but the deaths could be due to anything from toxic algae to chemicals and pesticides.

Sunderman said there were no leads yet on which of potentially hundreds of toxins might be responsible. Some birds were seen convulsing as they died.

Michelle Crisp was one of the first to contact the DEC after finding dozens of dead birds on her property one morning.

She told The Australian she normally had hundreds of birds in her yard, but that she and a neighbour counted 80 dead birds in one day. "It went to the point where we had nothing, not a bird," she said. "It was like a moonscape, just horrible.

"But the frightening thing for us, we didn't find any more birds after that. We literally didn't have any birds left to die."

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