The discovery comes a year after the mysterious mass death of 200 birds only a few kilometres away and two years after the Esperance lead contamination scandal which emptied the skies over the holiday town of Esperance for months when thousands of birds were poisoned.
The Department of Environment and Conservation yesterday blamed the latest deaths on the pesticide Fenthion, but said it was unclear whether it was a deliberate bird poisoning or had been caused by someone illegally dumping pesticide.
It is an active ingredient in a number of products sold at hardware stores, including fly baits and fruit fly sprays. It was not known yesterday how much poison would have been needed to cause so many deaths.
A DEC spokeswoman said if the birds were deliberately poisoned it was an offence under the Wildlife Conservation Act, with fines of up to $4000 for each bird species affected. Illegal dumping of pesticides was covered by the Environmental Protection Act.
In July, 200 seagulls died in Henderson and the neighbouring beach suburb of Woodman Point, sparking a major investigation. Authorities conducted dozens of autopsies and sent samples to laboratories across Australia but no cause of death was established.
In 2007, a catastrophic bird kill in Esperance, 725km southeast of Perth, silenced the dawn chorus for months when up to 10,000 wattle birds, yellow-throated miners, honeyeaters and other species died. The drama was the first warning of a lead contamination scandal which was later found to have caused dangerously high lead levels in children.