Household rat poison is being blamed for the demise of the common boobook owl population, which is ironic because this raptor is a natural predator of vermin.

Boobook owls are named after their distinctive call and are the most common and widespread owls in Australia.

But worryingly their numbers have been in decline for the past 15 years.

Michael Lohr, who in the final stages of his PhD at Perth's Edith Cowan University, has been investigating the role rodenticide poisons may be playing in the owl's decline.

As part of his research he put the call out for people and organisations to send him dead owls from around Perth and the Wheatbelt.

He ended up doing necropsies on 73 owls.

"It's not the most pleasant work but we are learning very interesting things from it," Mr Lohr said.

"More than 70 per cent of the boobooks that I tested had measurable levels of rodenticide and about 18 per cent had lethal levels.

"Another 33 per cent had levels that were in the dangerous-to-lethal range."

Michael Lohr has been studying boobook owls as part of his PhD. ( Supplied: Ben Jones )

Even a lower level of poison in their systems could lead to owl deaths, as the birds might react and move more slowly.

"A lot of birds tested were roadkill," he said.

"[The poison] may contribute to road death or being attacked by cats or dogs."

Rat poison spreading through the food chain

Anti-coagulant rat poison is widely used in Australia and is highly effective at bringing the vermin under control.

The poison works by accumulating in the body, blocking the recycling of vitamin K in the liver and causing rats to bleed to death after about a week.

"The problem is that there are two different kinds of poison," Mr Lohr explained.

"There are the first-generation poisons, like Warfarin, that were synthesised back in the '40s to the '60s and those ones actually break down reasonably quickly.

"In the '70s and '80s, because rodents started to develop resistance, they developed a second-generation product and these ones are much more persistent.

"[The poisons] accumulate faster than they are breaking down in a lot of predatory animals that eat the rats and as a consequence they get poisoned."

Boobook owls are natural predators of rats but often ingest rat poison as a result. ( ABC News: Irena Ceranic )

Changing your poison or revert to old-fashioned traps

There are several solutions for householders who want to be rid of rats without harming boobooks, and one may be to go back to using the first-generation poisons containing warfarin.

"Warfarin is certainly a lot better for wildlife and it's not known if the resistance issues exist here in Perth," Mr Lohr said.

But the simplest solution might be to control rats using more old-fashioned methods.

"Some of the best ways to control rats is just cleaning up the place a bit," Mr Lohr said.

"Keeping pet food indoors.

"Black rats love to live in palm trees, so take them out and replace them with native vegetation which is great habitat for things that control them naturally.

"Just reduce trash, seal holes in your house — that does a real number on rats.

"Even the old-fashioned snap traps are still really good.

Michael Lohr suspects other species are probably worse off than boobooks when it comes to poisoning. ( Supplied: Simon Cherriman )

"It would be a bit ironic if we are losing some of our natural rodent control in response to attempting to control rodents."

The scheduling of second-generation rat poisons is being reviewed by the Australian Pesticides and Veterinary Medicines Authority, a move welcomed by Mr Lohr.

"The US and Canada have both brought in much more serious restrictions on second-generation poisons, partly because of the impact on wildlife, but also because they were really common sources of poisoning in children and pets," he said.

In Australia, they can be bought in a local hardware store but "there is a question about whether things need to be brought into line with other countries — with requirements for lockable bait boxes and indoor use only by licensed pest controllers", Mr Lohr added.