Powerful owl deaths fuel concerns mouse poison is spreading through food chain

In a chest freezer in Beth Mott’s office in Wollongong are the bodies of 40 powerful owls – cause of death as yet undetermined.

Over the past 18 months, the dead specimens have been coming in from across Sydney and as far north as Newcastle, west to the Blue Mountains and south to Shoalhaven.

“I can’t fit another dead owl in there,” says Mott, of BirdLife Australia.

Autopsies on some of the birds – Australia’s biggest owl, with a wingspan of 1.4 metres – are pointing to a worrying conclusion. Many have suffered internal bleeding. Tiny wounds that should have healed seem instead to have proven deadly.

Mott is trying to raise funds to test the livers of the owls. She thinks the birds have eaten rats or mice killed by shop-bought rodenticides known as second-generation anticoagulant rodenticides, or SGARs

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With one feed these poisons thin the blood of the rodents, causing death. But the poison doesn’t go away and is instead passed on to whatever animal eats the sick or dying mouse or rat.

Scientists in Australia have started to find these SGARs are now spreading widely through the food chain.

What can start as an attempt to rid a suburban backyard of mice and rats with products plucked from the shelf of a hardware store can cascade through the food chain to threaten native animals and apex predators.

“We’re at the beginning of understanding all of this in Australia,” Mott says.

One of those SGARs is bromadiolone. The poison is the active ingredient in many off-the-shelf products and is approved for use in urban and rural areas to control rodents in and around buildings. The poison can remain active for about six months in the dead body of an animal.

The New South Wales government has secured 10,000 litres of bromadiolone and has applied for emergency permits to use it to treat grain to control the devastating mouse plague in the state’s west. More than 400 farmers have signed up in advance to get their grain treated.

The Australian Pesticides and Veterinary Medicines Authority (APVMA) is yet to decide if it will grant emergency permits. It said in a statement: “There are currently no bromadiolone products approved by the APVMA for use in crops.”

BirdLife Australia wants the permits denied. The charity also wants the sale of SGARs banned to the general public, and tighter restrictions and reporting when they are used by professionals.

Ornithologist Dr Maggie Watson, of Charles Sturt University, has a property near Albury in southern NSW just outside the worst-hit areas of the mouse plague. She says farmers should stick to using the zinc phosphide poison that acts within hours, rather than bromadiolone that can take up to 19 days to kill a rodent.

“Once it’s out in the field, you have a whole bunch of dead and dying rodents, as well as lizards that will eat it.

“It will leach into the soil. You will fill the food chain with this chemical and that’s going to decimate the landscape.”

Watson points to an incident in Mongolia in 2003 when hundreds of birds and animals died, including the endangered Saker falcon, after the government dispersed grain laced with bromadiolone across pastureland to control an outbreak of voles.

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The dangers of SGARs to “non-target” species have been known for years around the world but Dr James Pay, from the University of Tasmania, says Australia is only just discovering their dangers.

Pay carried out a study, published last month, that found SGARs in three-quarters of the recovered bodies of 50 Tasmanian wedge-tailed eagles, an endangered species.

What’s worrying, he says, is that these eagles are apex predators. They don’t eat many mice or rats but do eat larger animals that may have fed on poisoned rodents.

Pay says the eagles likely got the poison into their system after preying on animals that had suffered secondary poisoning in urban and agricultural areas.

“[In Australia] we are behind in the research but also in our understanding of the impacts,” he says. “We’re also behind a lot of places in Europe and the US where these second-generation anticoagulants are not available over the counter.”

Bill Bateman, an associate professor at Curtin University in Western Australia, has also been studying the levels of SGARs in three urban snake species around Perth, including the brown snakes that like to hang around the suburbs eating mice and rats.

Some 90% of the brown snakes, 60% of the bobtail snakes and 45% of tiger snakes had levels of SGARs in their livers.

Bateman expected to find SGARs in brown snakes that eat mice, but not the tiger snakes that mostly eat frogs.

“The problem is that reptiles cope quite well with these poisons and can still be moving around. Once it dies it’s then a toxic time bomb just waiting to be eaten and that will kill the other animal.”

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In 2018, one of the first studies looking specifically to test for SGARs in Australian wildlife found that more than 70% of southern boobooks – Australia’s smallest owl and most common owl – had at least one rodenticide in their livers. Some 38% had a cocktail, including SGARs.

Bateman says that because boobooks regularly fly long distances – up to 200km – “they can then be a source of toxins far away from where those second-generation anticoagulants were dropped”.

What is a bitter irony, he says, is that the SGARs that are deployed to control mice and rats are threatening the very birds that could help keep rodent numbers down.

“One of the brown snakes we tested had a level [of SGAR] in its liver that was three times the lethal dose for a bird of prey. We know a dead snake will be carried off by a harrier or a raven. If they eat the liver, they face the prospect of dying immediately.”

A statement from APVMA said instructions for using SGAR products, including bromadiolone, “can be accessed on the APVMA’s PubCRIS database, include restraints to control how the baits are used and mitigate risk to non-target species”.