California’s restrictions on lethal second-generation anticoagulant rodenticides are successfully driving down exposure rates among wildlife in northwestern forests, according to a peer-reviewed study published September 25, 2026, in the journal Environmental Pollution. Researchers from the Integral Ecology Research Center (IERC) analyzed liver tissue from 1,003 Barred Owls collected between 2019 and 2025. They found that exposure among young owls in the study area dropped from roughly 50 percent in 2019 to about 20 percent in 2024, offering some of the clearest evidence yet that state policy changes are delivering measurable real-world conservation benefits.
How California’s Strict Rodenticide Laws Drove Down Anticoagulant Exposure
The steep drop in wildlife contamination lines up closely with phased statewide regulations targeting toxic pest control products. Lawmakers enacted the California Ecosystems Protection Act (AB 1788) in 2020 to restrict second-generation anticoagulants. A subsequent 2024 law, AB 2552, extended those protective measures to older-generation compounds. Dr. Greta M. Wengert, co-founder and co-director of IERC and principal investigator on the study, noted that tracking these trends proves policy works when it rests on rigorous field data.
“We’ve watched illegal use and unregulated products drive rodenticide exposure in these forests for more than a decade,” Wengert said regarding the long-term monitoring effort. “Seeing anticoagulant exposure actually decline within a few years of California’s regulations taking effect is proof that policy, when it’s grounded in good data from effective biomonitoring efforts, can improve outcomes for wildlife.”
Product Substitution Sparks Fresh Concern Over Cholecalciferol Vitamin D3
While second-generation anticoagulant levels fell, researchers uncovered a troubling shift toward alternative toxins. The IERC team screened the Barred Owl liver samples for cholecalciferol, or vitamin D3, for the first time in this species. The compound turned up in 18.5 percent of the owls tested. Detections of vitamin D3 rose during the exact same window that anticoagulant poisonings declined, pointing to a clear pattern of product substitution as older chemicals became harder for consumers and commercial operators to buy.
Context on Total Wildlife Exposure
When researchers tallied all rodenticide compounds combined across the 1,003 specimens, roughly 46 percent of the sampled Barred Owls carried at least one type of toxin in their biological systems. Older adults and female owls bore the heaviest toxic load, accumulating diverse chemical mixtures over years of hunting in contaminated forest food webs.
Risks for Threatened Northern Spotted Owls and Pacific Fishers
The contamination patterns among Barred Owls raise serious alarms for vulnerable native species sharing the same habitat. Northern Spotted Owls rely on a higher proportion of small mammals in their diets than Barred Owls, exposing them to even greater dietary risks. Earlier research by IERC documented rodenticides in 70 percent of sampled Northern Spotted Owls in the region back in 2018, following a 2012 study that linked widespread Pacific fisher poisonings to trespass cannabis cultivation on public lands.
Dr. Vitek Jirinec, lead author of the study and an IERC research scientist, emphasized that the new dataset provides an exceptionally accurate snapshot because the animals were collected through an ongoing regional conservation program rather than picked up as dead or dying carcasses.
“Nearly all rodenticide surveys rely on animals that turned up sick or dead, so they tend to capture the worst cases,” Jirinec said. “Our owls were apparently healthy birds going about their lives. That gives us a much truer picture of what’s circulating in the population, and more confidence that the trends we see are real.”
Hidden Contamination Sources Persist Across Public and Tribal Lands
Despite strict new commercial rules, toxic compounds continue cycling through old-growth ecosystems from unregulated origins. State pesticide-use databases only log licensed commercial applicators, leaving household purchases and illicit activities entirely unmonitored. IERC has documented trespass cannabis cultivation on public and tribal lands as a major driver of forest contamination for more than a decade. Legacy poisons lingering at abandoned illegal grow sites continue to contaminate the food web long after active cultivation stops.
Laws and chemical shifts driving rodenticide exposure in owls
What specific laws drove the drop in rodenticide exposure?
The decline aligns with the California Ecosystems Protection Act (AB 1788), passed in 2020, and AB 2552, a 2024 law that extended restrictions to older-generation pest control compounds.
What new toxic chemical did researchers find in the owls?
Researchers screened for cholecalciferol (vitamin D3) for the first time and detected it in 18.5 percent of the Barred Owls, revealing a trend toward product substitution as older poisons faced tighter rules.
Why are female and older adult owls most affected?
Older birds accumulate multiple toxic compounds in their liver tissue over a lifetime of feeding on contaminated rodents in the forest food web.
How does this study differ from previous wildlife toxicology research?
Most prior studies tested animals found sick or dead. This research examined 1,003 apparently healthy birds removed through a regional conservation program, offering a representative population-wide baseline.
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