Mystery as thousands of birds fall from sky in Australia

Thousands of birds have fallen from the skies over Esperance 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, 725km southeast of Perth, was declared a natural disaster zone.