Beyond rodents: Apex predator diet reveals possum-mediated rodenticide poisoning

Anthropogenic modification of natural environments increasingly exposes wildlife to novel threats, including secondary poisoning from anticoagulant rodenticides (ARs). Second-generation ARs (SGARs), widely used globally, bioaccumulate in non-target species, posing a significant risk to apex predators. In Australia, the powerful owl (Ninox strenua), a large forest owl listed as vulnerable in Victoria, shows widespread SGAR exposure, yet their precise exposure pathways are unclear. While it's often assumed raptors acquire SGARs from consuming poisoned rodents, powerful owls historically prey predominantly on arboreal marsupials, with rodents comprising a very minor dietary component. Critically, key marsupial prey species, such as common ringtail (Pseudocheirus peregrinus) and common brushtail possums (Trichosurus vulpecula), have recently been shown to be highly exposed to SGARs. Using regurgitated pellets containing prey remains, this study investigated powerful owl diet across Victoria, Australia, from 2016 to 2024, to determine if a dietary shift towards rodents has occurred and to elucidate likely SGAR exposure pathways. 1225 prey items were identified across 1094 pellets, and show that powerful owl diet remains dominated by mammals (95 % of prey occurrences), with common ringtail possums, common brushtail possums, and Krefft's gliders (Petaurus notatus) constituting 92 % of prey occurrence. Importantly, we found no evidence of a widespread dietary shift towards rodents. We found a clear temporal trend towards increased Krefft's gliders and decreased possums in the diet, however, shifts in rodent consumption did not occur to any significant degree. Our results strongly indicate that secondary poisoning in powerful owls is primarily through the consumption of SGAR-contaminated native marsupials, rather than via an increase in rodent predation. Given the widespread public availability of SGARs in Australia, our findings further highlight an urgent need for tighter regulatory controls to mitigate this pervasive threat to native wildlife.