Research into the causes of death of large owls (Ninox strenua) in Melbourne indicates that secondary poisoning from anticoagulant rodenticides is much higher than previously thought.
We are well aware that the anticoagulant rodenticides used to control rodents have the potential to cause secondary poisoning in wildlife. The potency of the second-generation rodenticides (SGARs) such as brodifacoum, bromadiolone and difenacoum has proven to result in increased secondary poisoning events when compared to the first-generation rodenticides. The detection of these rodenticides in large birds that prey on rats and mice has been reported widely around the world.
Powerful owls, however, do not generally prey on rodents – so it would be logical to assume that they are at a very low risk of secondary poisoning. However, new research from a team of Australian academics suggests that powerful owls are not only susceptible to SGAR poisoning, but the incidence of secondary poisoning may be on the increase.
Researchers from Deakin University and The University of Melbourne launched their research following the reported death of eight powerful owls (Ninox strenua) around Melbourne in less than one year (2020-2021).1 This was added to the remains of ten powerful owls that had been collected between 2004 and 2019. The sharp rise in the number of dead owls prompted the researchers to undertake post-mortem examinations to determine if anticoagulant rodenticides (ARs) and/or agricultural chemicals were a factor in the deaths.
The findings were concerning. Anticoagulant rodenticides were detected in 15 of 18 owls (83.3%) examined in this study. Five of the ARs analysed were not detected in any samples (warfarin, coumatetralyl, difenacoum, difethialone and flucoumafen). Ten had a single AR detected in their livers, four had two types of AR, and one owl had three different rodenticides detected in its liver.
The most frequently detected rodenticide was the SGAR brodifacoum, which was detected in all 15 owls that had detectable rodenticide concentrations. Brodifacoum was often present at toxic levels and in some instances at potentially lethal levels. In fact, 11 of the 18 powerful owls (61.2%) examined had concentrations of SGARs over the level at which toxic effects may be expected to occur (Lohr, 2018).
Furthermore, the presence of brodifacoum was detected across the complete urban forest/agriculture gradient, suggesting widespread exposure. The question here is how are these owls suffering from secondary poisoning when they do not typically eat ground-dwelling prey, such as rodents?
Given that powerful owls prey primarily on tree-dwelling possums, the high prevalence of SGARs detected in this study suggests that these owls may be contaminated through consuming possums that have been exposed to rodenticides. Brodifacoum, the SGAR most frequently detected in the powerful owls, is the active ingredient in most retail rat poisons available throughout Australia. Although rodenticides are primarily used to control rodents, non-target possums may also consume this bait. The researchers suggested that this could be happening through the intentional poisoning of possums using rodenticides (an illegal act) or accidental poisoning through the incorrect use and/or placement of rodenticides.
Like many top-order predators, powerful owls lead long lives and population numbers are low. This means that any threatening processes, such as poisoning with toxicants, could have significant ramifications for the viability of owl populations in urban and semi urban landscapes. Even if not lethal or acutely toxic, SGARs would have some impact on their overall health, fitness, and reproductive function.
The findings are not only in Melbourne; in 2018, a study2 into AR exposure rates in Australian wildlife found that ARs – the kind used only by licensed pesticide applicators – were detected in 72.6% of Southern Boobook owls found dead or moribund in Western Australia. Although in this study, the level of exposure to ARs was greater in developed habitats rather than widespread across all environments.
The researchers believe the findings of the study firstly highlight a critical need to investigate SGARs in food webs globally, and not just in species directly targeted for poisoning or their predators. Secondly, as in other parts of the world, increased regulation of the sale of SGARs in Australia should be introduced with immediate effect.
BirdLife Australia Urban Bird Program manager, Dr Holly Parsons, believes the results of the study provide more evidence that we need to stop selling second generation rodenticide products to consumers.
“The samples tested in this research were largely collected prior to this year’s mouse plague, so what we are seeing is indicative of contamination from baits used by householders during normal periods,” she commented. “I shudder to think what impact might be playing out in our native wildlife now from eating rodents that were baited during the plague.”
This piece of research seems timely, following the APVMA public consultation into use patterns for anticoagulant rodenticides, which took place in 2020. Following the assessment, the APVMA has commenced a chemical review of ARs and determined that they “present a higher level of risk to humans and non-target animals (such as predatory birds) than would normally be considered acceptable… SGARs must be removed from the shelves for public sale and only made available to licenced professional operators only”.3
1 Raylene Cooke, Pam Whiteley, Yun Jin, Clare Death, Michael A. Weston, Nicholas Carter, John G. White, Widespread exposure of powerful owls to second-generation anticoagulant rodenticides in Australia spans an urban to agricultural and forest landscape, Science of The Total Environment, Volume 819, 2022, 153024, ISSN 0048-9697.
2 Lohr, Michael. (2018). Anticoagulant rodenticide exposure in an Australian predatory bird increases with proximity to developed habitat. The Science of the total environment. 643. 10.1016/j.scitotenv.2018.06.207.
3 Australian Pesticides and Veterinary Medicines Authority 2020. Summary of submissions to the public consultation on use patterns for anticoagulant rodenticide products. September 2020.