A common farm chemical is believed responsible for the deaths of hundreds of protected sulphur-crested cockatoos in Victoria's northeast.
And investigators believe the chemical, often used to control mites, had possibly been made into baits.
More than 250 of the birds died at Tatong, near Benalla, in January and February, prompting calls to wildlife officers from the Department of Environment.
Greg Chant from the department said testing of samples from the dead birds indicated they had died from omethoate poisoning.
"Omethoate is a common farm chemical used to protect crops from red-legged earth mites," he said.
"It's possible omethoate was illegally used to create a homemade bait, which the birds ate."
The cockatoos are protected under the Wildlife Act and there are significant penalties - including imprisonment - for hunting, taking or destroying protected species.
It is also illegal to make bait products without appropriate authorisation, Mr Chant said.
"The incident highlights that using chemical products in an illegal way poses an unacceptable risk to wildlife," he added.
"It is unclear if the birds were deliberately poisoned or not, but illegally destroying protected native wildlife is a serious environmental crime."
The department is now looking for the person responsible and anyone with information can make anonymous calls to the department on 136 186 or Crime Stoppers on 1800 300 000.