Rat Poison Found in Half of Dead Birds of Prey in Queensland Study

A quiet chemical crisis may be unfolding in the skies over eastern Australia. When researchers at The University of Queensland examined the bodies of 24 predatory birds collected around south-east Queensland and northern New South Wales, more than half of them carried anticoagulant rodenticides in their livers, the potent rat and mouse poisons designed to kill rodents but increasingly implicated in the deaths of the very predators that keep those rodents in check. The study, published in December 2024 in the journal Discover Toxicology, offers one of the most detailed looks yet at sublethal rodenticide exposure in Australian birds of prey, and its findings carry uncomfortable implications for how the nation manages its periodic mouse plagues.

The birds in the study were not killed for research. All 24 cadavers arrived opportunistically between June 2022 and November 2023, a window that coincided with regional mouse infestations. Some were donated by the university’s Veterinary Teaching Hospital after being euthanised for medical reasons, while others were roadkill collected during unrelated fieldwork. Ten raptor species and one nightjar were represented, with Eastern Barn Owls dominating the sample at ten individuals, followed by three Australian Boobook Owls. Two Wedge-tailed Eagles, two Black-shouldered Kites, a Peregrine Falcon, a Powerful Owl, a Grey Goshawk, a Brown Goshawk, a Nankeen Kestrel, a Black Kite and a Tawny Frogmouth rounded out the list. Thirteen birds were male, ten female and one of undetermined sex; fourteen were adults and ten immature.

The analytical centrepiece of the study was liquid chromatography mass spectrometry, or LC-MS, performed at the CSIRO Ecosciences Precinct in Brisbane. Because decay alters the water content of tissue, the researchers homogenised and freeze-dried liver samples before extraction, then used a multi-step process involving formic acid, cold acetonitrile and lipid-removal cartridges to isolate the rodenticide residues. The instrument, an Agilent 6546 liquid chromatography quadrupole time-of-flight mass spectrometer, ran in a data-independent mode called quadrupole-resolved all-ions analysis, screening for eight rodenticides including warfarin, brodifacoum, difethialone, flocoumafen, bromadiolone, difenacoum, coumatetralyl and pindone. Calibration curves spanning 0.01 to 1,000 parts per billion achieved correlation coefficients between 0.9683 and 0.9971, and quality-control samples spiked with known concentrations of the compounds verified the method’s accuracy across low, mid and high ranges.

The results were striking. Thirteen of the 24 birds, just over half, tested positive for rodenticides registered for use in Australia. The most frequently detected compound was warfarin, a first-generation anticoagulant, found in eleven birds. Second-generation anticoagulants, which are far more toxic and persistent, also appeared: brodifacoum in three samples, difethialone in one and flocoumafen in one. Three birds carried warfarin alongside a second-generation compound, meaning they had been exposed to multiple poisons. Perhaps most telling, only six of the thirteen rodenticide-positive birds had rodent remains in their digestive tracts at the time of death. The researchers interpret this as evidence that the poisons can bioaccumulate in liver tissue over time, lingering from past meals of poisoned prey rather than reflecting a single recent exposure.

None of the detected concentrations reached the lethal dose thresholds established for other bird species. Brodifacoum levels in the Barn Owls and a Wedge-tailed Eagle fell below the reported oral LD50 range for owls of 0.15 to 9.68 milligrams per kilogram, and the difethialone concentration in the Tawny Frogmouth, 0.1 milligrams per kilogram, sat below the LD50 values of 0.26 to 23.4 milligrams per kilogram documented for Bobwhite, Japanese Quail and Mallard. Warfarin concentrations were similarly far below the 525 to 2,150 milligrams per kilogram lethal range reported for those same test species. But sublethal does not mean harmless. Second-generation anticoagulants are highly lipid-soluble and resist metabolic breakdown, and brodifacoum has been shown to cause persistent coagulopathy in birds for up to eleven days even at doses below the lethal threshold.

The clinical consequences of such exposure can be subtle but deadly. Birds suffering sublethal anticoagulant toxicosis have been documented to show lethargy, reduced agility and wing droop, impairments that degrade hunting performance and spatial navigation. For a fast-flying owl hunting at night, a momentary lapse in coordination can be fatal in a different way: collision with a vehicle. Twelve of the fifteen birds in the study showing traumatic injuries were found on roadsides, and the authors suggest that some of these roadkill birds may have been compromised by sublethal poisoning before they ever met a car. Peak coagulopathy after a single lethal ingestion typically occurs one to four days later, and after sublethal ingestion the effects may take even longer to manifest, widening the window of vulnerability.

Disentangling poisoning from trauma is one of the study’s central methodological challenges. Internal bleeding looks much the same whether it stems from a blunt impact or a drug-induced failure of the clotting cascade, and decomposition or scavenging can destroy clots that would otherwise distinguish the two. Ten birds showed haemocoelom, a coelomic cavity filled with uncoagulated blood, and in six of those there were no grossly obvious external signs of trauma such as fractures or lacerations. Yet only four of the ten haemocoelom cases were rodenticide-positive, and the statistical association between anticoagulant presence and postmortem evidence of coagulopathy was not significant. Alternative causes of bleeding disorders complicate the picture further: three birds carried low-burden apicomplexan parasites consistent with avian malaria, two had chronic enteritis with coccidian oocysts possibly of the genus Goussia, and heavy metal toxicity and bacterial infections such as salmonellosis can produce similar pathology in wild raptors.

One statistically significant pattern did emerge, and it concerned age. Immature birds were significantly more likely to carry higher brodifacoum concentrations than adults, a correlation with an f-value of 5.15 and a p-value of 0.03. The authors offer several non-exclusive explanations: inexperienced young hunters may target weakened, poisoned prey more readily; older birds may metabolise or eliminate the compound more efficiently or simply have longer intervals between exposures; or heavily affected juveniles may never survive to adulthood at all. The researchers caution, however, that the small sample size and a confidence interval hovering near zero mean the finding should be treated as a hypothesis generator rather than a settled conclusion. Species-level dietary differences add another layer of complexity, since rodent specialists such as Barn Owls and Black-shouldered Kites face theoretically higher exposure risk than bird-eating specialists like goshawks.

The conservation stakes extend beyond the species examined, all of which are currently listed as Least Concern by the International Union for Conservation of Nature. The Powerful Owl, despite its global status, numbers only an estimated 2,200 to 2,800 mature individuals, and the Grey Goshawk is estimated at fewer than 10,000 birds with a declining trend and Endangered classification in several Australian states. Sublethal poisoning that reduces foraging success, body condition and reproductive output, effects documented in vulture studies in the Pacific Northwest, could push such populations toward localised declines long before outright poisoning deaths register. Australian raptor populations remain poorly studied overall, making it difficult to assess how much hidden damage rodenticides are doing.

The study’s most policy-relevant finding may be its challenge to current Australian guidelines, which favour first-generation rodenticides such as warfarin over second-generation products on the assumption that they break down faster in rodents and pose less secondary risk. Warfarin was, after all, the most common residue in these Queensland birds. The authors point instead to coumatetralyl, another first-generation anticoagulant regarded as safer for predatory birds: mammals are generally more sensitive to it than birds, allowing lower effective doses, and while warfarin resistance has been documented in Australian rodents, no coumatetralyl resistance has yet been recorded there. The team calls for physiological studies of exposed raptors, rodent toxicity testing to track resistance, and surveys of rodenticide users to map what, how much and how often is being deployed. Until then, the poisons meant for mice appear to be quietly accumulating in the livers of the owls, eagles and falcons that share those fields, a debt the ecosystem may only now be beginning to itemise.

Subject of Research: Sublethal anticoagulant rodenticide exposure in deceased predatory birds in south-east Queensland, Australia

Article Title: Evaluating sublethal anticoagulant rodenticide exposure in deceased predatory birds of South-East Queensland, Australia

Article References: Low, Z., Murray, P. J., Naseem, N., McGilp, D., Doneley, B., Beale, D. J., Biggs, L., & Gonzalez-Astudillo, V. (2024). Evaluating sublethal anticoagulant rodenticide exposure in deceased predatory birds of South-East Queensland, Australia. Discover Toxicology, 1(1), Article 13. https://doi.org/10.1007/s44339-024-00016-4

Image Credits: AI Generated

DOI: 10.1007/s44339-024-00016-4

Keywords: anticoagulant rodenticides, raptors, owls, warfarin, brodifacoum, secondary poisoning, wildlife toxicology, LC-MS, Australia, Queensland, coagulopathy, conservation