A major phenomena has occurred over the West Australian coastal town of Esperance.

Several thousands of birds, of many different species, have mysteriously dropped dead out of the sky.

Investigations by scientists and vetinarians in the West Australian capital of Perth have failed to discover the cause of the mass deaths.

The Australian newspaper says 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, say The Australian, are baffled by the 'catastrophic' event, which the Department of Environment and Conservation said began well before a freak storm last week.

On Monday, Esperance, 725 kilometres southeast of Perth, was declared a natural disaster zone.

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,' Mr 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.'

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

Acting chief veterinary officer Fiona 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.'