A long-term study has found high levels of rat poison in wedge-tailed eagles, suggesting that the toxins are having a broad impact on the food chain.
- Rat poison has been found in 74 per cent of dead wedge-tailed eagles in Tasmania
- A study finds the eagles do not usually eat rats and mice, so the poison must be coming from animals that eat rodents
- Other large birds and mammals such as quolls and Tasmanian devils are also at risk
The University of Tasmania research, spanning more than two decades and released in May this year, is the first systematic study of rat poisons in an Australian top predator.
It identified poison in 74 per cent of 50 dead wedge-tailed eagles found in Tasmania.
Concentrations of brodifacoum, a common rat poison which causes internal bleeding, was four times higher than previously reported in eagles overseas.
Study leader James Pay said wedge-tailed eagles did not normally prey on mice and rats, so the poison must be entering their systems by other means.
“So a rat eats the poison, and then things like forest ravens eat the rat, and then the forest raven gets sick or dies, and then a wedge-tailed eagle eats the forest raven,” he said.
Second-generation poisons in the mix
The study also implicated professional pest control and agriculture.
High concentrations of rat poison were found in eagles closest to agricultural land while flacoumafen – a poison only available through wholesalers – was found in nearly half of the sampled birds.
“To have such high exposures to flacoumofen suggests that there’s a link between agricultural use and these [poisons] ending up in wedge-tailed eagles,” Dr Pay said.
Birdlife Australia raptor specialist Nick Mooney said a big part of the problem stemmed from the use of second-generation poisons.
“The first-generation [poisons] aren’t as powerful as the second generation,” he said.
“Second-generation just means the much newer, more aggressive poisons, and they’re often advertised as ‘single-dose only’ — those sorts of claims.
“Because these poisons take a while to kill the rodents, the rodents can … eat far more poison than they actually need to kill themselves.
Not just eagles
Mr Mooney warned wedge-tailed eagles were not the only animals affected by rat poisons — birds and animals that ate poisoned rats and mice like hawks, owls, magpies, tawny frogmouths, quolls and Tasmanian devils were all at risk.
“There’s nothing quite so ubiquitous as a …food as a mouse or a rat,” he said.
The news comes as the mouse plague in eastern Australia enters its eighth month, and as the New South Wales government seeks approval from the Australian Pesticides and Veterinary Medicines Authority (APVMA) to distribute 10,000 litres of bromadiolone to farmers across the state.
Described as “napalm” for mice, Dr Pay believes the untargeted use of second-generation poisons like bromadiolone could spell disaster, not just for wedge-tailed eagles, but anything that eats rodents or the baits used to kill them.
“It’s pretty concerning,” he said.
Only ‘scratching the surface’
Bonorong Wildlife Sanctuary treats sick and injured raptors, including wedge-tails, at least once a week. Greg Irons owns and runs the business and said secondary poisoning was an issue largely hidden from sight, and therefore from mind.
“We certainly find when there’s an increase in mice there’s an increase in poisoning cases as well,” he said.
“I can’t stress enough, I think we are scratching the absolute surface when we say, ‘OK, it’s only X amount per year,’ those are only the ones we know about and it is much, much higher than any of us wish to think.”
Mr Irons said it was a matter of educating those who used rat poisons.
“I would say pretty confidently that 90 per cent of people that I’ve talked to about this issue hadn’t really considered secondary poisoning, but are very aware of baits being potentially very dangerous,” he said.
Mr Mooney agreed rodenticides were an effective management tool and did have a role to play on the farm, but their use should be limited to first-generation poisons.
“When there’s a proper plague and there are seriously high numbers, yes, chemicals are often necessary, but we’re just asking people to think hard about it and go for the least harmful poisons.”