California’s Rodenticide Crackdown Is Working, Seven-Year Owl Study Finds, but a New Threat Emerges

In one of the most consequential wildlife toxicology studies ever conducted in North America, researchers have documented a striking decline in the exposure of wild owls to the deadliest class of rat poisons following California’s landmark restrictions, while simultaneously raising the alarm about a less-regulated replacement compound that is quietly spreading through forest food webs. The study, led by the Integral Ecology Research Center (IERC) and published in the journal Environmental Pollution, analyzed liver tissue from 1,003 Barred Owls collected across northwestern California between 2019 and 2025, making it the largest rodenticide biomonitoring dataset ever assembled for a single wildlife species on the continent. The findings offer something exceedingly rare in conservation science: direct, quantifiable evidence that legislation, when grounded in rigorous field data, can measurably reduce contamination in wild animal populations within just a few years.

The scale and design of the study are what set it apart from previous work. Most rodenticide surveys rely on animals that are found sick, dying, or dead, which means the samples skew toward the worst cases and overestimate exposure in the broader population. This new research took a fundamentally different approach. The owls in the dataset were removed as part of an ongoing regional conservation program targeting Barred Owls, an invasive species that competes with the threatened Northern Spotted Owl, rather than recovered as carcasses. As a result, the sample represents apparently healthy birds going about their normal lives, providing a far more accurate cross-section of what is actually circulating through the population. Dr. Vitek Jirinec, lead author of the study and research scientist at IERC, emphasized that this distinction gives researchers much greater confidence that the observed trends are real and population-wide rather than artifacts of sampling bias.

The headline result concerns second-generation anticoagulant rodenticides, or SGARs, an extremely potent class of compounds that prevent blood clotting and can kill predators through a single feeding on a poisoned rodent. Because these chemicals persist in the tissues of poisoned prey, they bioaccumulate up the food chain, devastating owls, fishers, bobcats, mountain lions, and other carnivores that never touched the original bait. Among young owls sampled in the study area, exposure to SGARs fell from roughly 50 percent of birds in 2019 to about 20 percent in 2024. That steep decline aligns closely with California’s phased statewide restrictions, most notably the California Ecosystems Protection Act, known as AB 1788, which was passed in 2020 and banned most second-generation anticoagulant use, along with a 2024 law, AB 2552, that extended restrictions to older-generation anticoagulant compounds as well.

For Dr. Greta M. Wengert, co-founder and co-director of IERC and principal investigator on the study, the results represent the culmination of more than a decade of work connecting contamination data to policy action. She noted that illegal use and unregulated products had driven rodenticide exposure in these forests for years, and that seeing anticoagulant exposure actually decline within a few years of the regulations taking effect demonstrates that policy grounded in good data from effective biomonitoring efforts can improve outcomes for wildlife. The study embodies what researchers describe as a feedback loop central to IERC’s mission: supplying the science that informs legislative decisions, and then documenting the real-world benefit that those decisions deliver on the ground.

But the study is not simply a victory lap. For the first time in this species, the researchers screened for cholecalciferol, commonly known as vitamin D3, a rodenticide that works through an entirely different mechanism than anticoagulants. Rather than disrupting blood clotting, cholecalciferol in toxic doses causes life-threatening hypercalcemia, raising calcium levels in the blood until organ failure occurs. The compound turned up in nearly one in five owls, at 18.5 percent of the sample, and detections rose over precisely the same period that anticoagulant exposure was falling. The researchers interpret this inverse pattern as evidence of product substitution: as older, regulated compounds became harder to access, users appear to have shifted toward cholecalciferol-based products, which currently face fewer restrictions in California.

The overall contamination picture remains sobering. When all rodenticide compounds were counted together, as much as 46 percent of the owls sampled carried at least one rodenticide in their system. This means that even as the most lethal anticoagulants recede, nearly half of the owls in the study area are still being exposed to some form of poison through the prey they consume. The data also revealed telling patterns within the population: female Barred Owls and older adults carried the highest exposure levels and the widest mix of compounds, a result consistent with pesticides accumulating in tissues over a lifetime of feeding on contaminated rodents.

That demographic pattern carries particular weight for conservation, because the individuals with the highest burdens, breeding-age females and established territory holders, are exactly the animals a population depends on most to sustain itself. The implications extend beyond Barred Owls to the species they share the forest with. Northern Spotted Owls, a closely related and federally threatened species, have a higher proportion of rodents in their diets, which may place them at even greater risk of accumulating these compounds. The new findings reinforce and extend earlier IERC research: a 2012 study first documented widespread anticoagulant exposure in Pacific fishers linked to trespass cannabis cultivation on public lands, and a 2018 study found rodenticides in 70 percent of sampled Northern Spotted Owls in the same region. Barred Owls, Spotted Owls, fishers, and other sensitive wildlife all feed from the same contaminated food web, and this much larger dataset adds substantial weight to that earlier evidence.

The authors are careful to caution that the exact sources of ongoing exposure are not fully resolved, and this uncertainty points to significant gaps in regulatory oversight. State pesticide-use records capture only licensed commercial applicators, leaving household purchases and illegal use entirely invisible to regulators. Trespass cannabis cultivation, which IERC has long documented as a major source of rodenticide contamination on public and tribal lands, remains untracked by any regulatory database. Legacy contamination from past illicit grow sites may also continue cycling through these forest ecosystems long after active cultivation has stopped, as persistent compounds move from soil and bait stations into rodents and then into the predators that hunt them. Researchers say these hidden and historical sources likely explain at least part of the exposure that persists even as legal, regulated use declines.

Published as Forest food web contamination by rodenticides is pervasive but responsive to regulation: Insights from Barred Owl biomonitoring, the study by Jirinec, Gabriel, Peery, Higley, Hofstadter, Varian, Rex, Franklin, Poppenga, Woods, Volker, and Wengert arrives at a pivotal moment for pesticide policy. It demonstrates that science-based regulation can produce measurable improvements for wildlife within a few years, a finding that should embolden policymakers elsewhere considering similar restrictions. At the same time, the rising tide of cholecalciferol detections serves as a warning that substitution effects can undermine hard-won gains when regulatory frameworks address only one class of compounds at a time. For the old-growth forests of northwestern California and the threatened species that depend on them, the message from more than a decade of IERC research is twofold: the same underlying toxic threat has persisted across multiple species, but for the first time, it is demonstrably responding to policy, and the next chapter of that policy response will determine whether the progress holds.

Subject of Research: Rodenticide contamination in Barred Owls and the effect of California pesticide regulations on wildlife exposure

Article Title: Largest-ever study of its kind shows California’s rodenticide restrictions are working, and flags new rodenticides rising in their place

Article References: Largest-ever study of its kind shows California’s rodenticide restrictions are working, and flags new rodenticides rising in their place. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: rodenticides, Barred Owls, California Ecosystems Protection Act, second-generation anticoagulants, cholecalciferol, Northern Spotted Owl, biomonitoring, wildlife toxicology, Integral Ecology Research Center, Environmental Pollution, pesticide regulation, forest food web