Horror mouse plague triggers unprecedented owl casualties

Mystery surrounds a dramatic spike in sick and injured barn owls being rescued across Australia’s eastern states.

Wildlife Victoria recorded one of the biggest increase in cases, with the rescue group attending to 20 of the birds in the week commencing July 25, with numbers surging to 27 in two seperate August weeks.

With between two and three individuals taken into care during corresponding weeks in 2019 and 2020, CEO Lisa Palma described this year’s surge as “extraordinary”.

An image of a sick owl on a towel against a pile of dead mice.
Researchers have linked the recent mouse plague to the spate of owl rescues. Source: WIRES / NSW Farmers

While large clusters of cockatoos and galahs are believed to have died this year from eating poison grain, the Victorian barn owl rescues have not been confined to one regional area, and have even occurred in metropolitan Melbourne.

“We’re most commonly called because they’re being attacked by another animal, or being found on the ground,” Ms Palma said.

“If a barn owl is found on the ground… it’s usually sick and not flying, and if it’s being attacked, that’s indicative of the fact that the barn owl is already compromised.”

Barn owl cases spread to more Aussie states

ACT Wildlife Vice President Marg Peachey said the charity has cared for more barn owls this year than ever, with approximately 75 needing assistance.

“Across the previous eight years, we've probably had a total of 10 or 11 combined,” she said.

“This is an extraordinary year and it's put a lot of strain on our carers.”

NSW rescue group WIRES also recorded an increase in barn owl rescues, with carers surprised to be finding them in areas where they are not regularly seen including Cronulla, Parramatta and Kurrajong Hills.

Volunteers across the Parkes region, in the western part of the state, attended to 51 barn owl callouts between June and September, compared to just one over the same period last year, and three in 2019.

Their branch's raptor coordinator, who has 15 years experience with the organisation, said she has never seen the species come into care in such significant numbers.

Two pictures of owls on window ledges.
Victorian rescuers have attended to "extraordinary" numbers of sick and injured barn owls. Source: Wildlife Victoria

South East Queensland's Currumbin Wildlife Hospital reported 22 barn owls in six weeks, with senior vet Michael Pyne saying most of their submissions were subadults being attacked by noisy miners, butcherbirds and magpies which become aggressive during breeding season.

In contrast, Bird Rescue South Australia and Western Australia's FAWNA said they have not recorded any discernible increase in rescues.

How mouse plague relates to barn owl illness a mystery

The surge is likely directly linked to the recent mouse plague, but how the event is affecting barn owls, which feed on the vermin, is yet to be determined.

Birdlife Australia’s Dr Holly Parsons said it’s unknown whether the the surge in rescues is primarily driven by increased breeding followed by starvation as rodent numbers decrease, rat bait poisoning, or an unknown disease.

Internationally, testing has shown barn owls are impacted by rodenticide, and Australian research has revealed boobooks and Tasmanian wedge-tailed eagles are also being significantly harmed.

"There's a range of 30-odd species of birds in Australia that have been shown to have anticoagulant rodenticides in them," Dr Parsons said.

“That’s only the birds, that’s not looking at marsupials and reptiles as well, it’s prolific right through the food chain.

“While it's hard to say without the definitive analysis of autopsies and liver testing, it's certainly possible that at least some of the barn owls we’re seeing are impacted by rodenticides.”

Why barn owl numbers surge during mouse plagues

Dr Parsons describes barn owls as a "boom and bust" species.

Most of the owls rescued around Canberra were rehabilitated and released. Source: ACT Wildlife
Most of the owls rescued around Canberra were rehabilitated and released. Source: ACT Wildlife

When food is scarce, only one or two barn owl chicks will survive out of a clutch of three and six eggs, but when food is plentiful, it’s common for all chicks to survive, and for the parents to lay successive broods.

With more barn owls around, there's a greater chance of them coming into care, and when the winter comes and the mice go into hiding starvation can occur.

Not the first mass barn owl mortality event

Two similarly significant barn owl mortality events in 2011 and 2018 have been observed by Sydney University’s Professor David Phalen, with both occurring after mouse plagues.

"It's clear that every time we have a mouse plague, we get a barn owl eruption, and we get increased numbers of them coming into care," Professor Phalen said.

“During the first two mouse plagues, we had barn owls come in and it became very obvious that they're not just young and dumb; these barn owls are sick.

“They're usually exhibiting neurologic signs, oftentimes they're out in the middle of the day, quite often there's a lot of haemorrhage in their bodies after you do a post-mortem on them.

“I'm convinced that a lot of these barn owls that are coming in now are sick from other reasons."

An owl in a cage. An owl on a log with a broken wing.
Owls have presented emaciated (right) and with breaks and injuries (left). Source: WIRES

Internal lesions on their livers and kidneys of owls examined during previous "boom and bust" events led Professor Phalen to suspect the deaths were driven by more than just starvation.

Owls impacted in 2011 showed signs of a type of rat bait which has high levels of vitamin D, while Professor Phalen hypothesises zinc phosphide, which is widely believed to be a less harmful form of rodent bait, could be behind some of the mortalities in 2018.

Owls autopsied in 2018 had "severely compromised immune systems" and small scarred up spleens, leading him to believe they may have also incurred a systemic viral infection at some stage.

"It's a complicated thing," he said.

"Maybe more than one thing is going on, or maybe a lots of things going on, it's hard to know."

What you need to know about how the mouse plague unfolded this year

What do do if you encounter clusters of dead owls

The NSW Environment Protection Authority said in recent months they have not received any reports of barn owl deaths.

"The EPA urges the community to report any instance of more than 5 dead animals of the same species immediately," a spokesperson said.

"If you suspect a native animal has been poisoned, do not touch it due to the risk of harm from exposure to the pesticide or the toxic phosphine gas that may be given off."

Anyone with information can contact the EPA 24-hours a day, seven days a week on 131 555 or email info@epa.nsw.gov.au.

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