BLUE LAKE, Calif. September 25, 2026 - New research from the Integral Ecology Research Center (IERC) offers some of the clearest evidence yet that California’s efforts to restrict dangerous rodenticides are paying off in the wild, while also flagging a new contaminant that deserves close attention going forward.
The study, published in the journal Environmental Pollution, analyzed liver tissue from 1,003 Barred Owls collected across northwestern California between 2019 and 2025, the largest rodenticide biomonitoring dataset ever assembled for a single wildlife species in North America. Because the owls were removed as part of an ongoing regional conservation program rather than recovered as sick animals or dead carcasses, the sample is a far more representative picture of the population as a whole.
The findings are a mix of a genuine policy win and a new warning.
The policy win: Among young owls sampled in the study area, exposure to the most lethal class of rodenticides, known as second-generation anticoagulants, fell from about 50% of birds in 2019 to about 20% in 2024. That decline lines up closely with California’s phased statewide restrictions, most notably the California Ecosystems Protection Act (AB 1788), passed in 2020, along with a 2024 law extending restrictions to older-generation compounds (AB 2552). It also reflects a feedback loop central to IERC’s work: supplying the science that informs policy decisions, and then documenting the real-world benefit that policy delivers within just a few years.
The new concern: At the same time, the study screened for cholecalciferol (vitamin D3), a different type of rodenticide, for the first time in this species. It turned up in nearly one in five owls (18.5%), and detections rose over the same period that anticoagulant exposure was falling, a pattern suggesting product substitution as older compounds become harder to access. Overall, when all rodenticides were counted, as much as 46% of the owls sampled carried at least one rodenticide compound in their system.
The Spotted Owl connection: Female Barred Owls and older adults carried the highest exposure and the widest mix of compounds, consistent with pesticides accumulating over a lifetime. These are exactly the individuals, breeding-age females and established territory holders, that a population depends on most to sustain itself. Because Northern Spotted Owls are a closely related species with a higher proportion of rodents in their diets, this pattern may indicate added risk for Spotted Owl recovery.
“Nearly all rodenticide surveys rely on animals that turned up sick or dead, so they tend to capture the worst cases,” said Dr. Vitek Jirinec, lead author of the study and research scientist at IERC. “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.”
Dr. Greta M. Wengert, co-founder and co-director of IERC and principal investigator on the study, has tracked rodenticide contamination across the region for more than a decade. She said the results demonstrate something rare in conservation science, a policy translating directly into a measurable outcome for wildlife:
“We’ve watched illegal use and unregulated products drive rodenticide exposure in these forests for more than a decade,” said Wengert. “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.”
The study also reinforces and extends findings from earlier IERC research. A 2012 IERC study first documented widespread anticoagulant exposure in Pacific fishers linked to trespass cannabis cultivation on public lands, and a 2018 IERC study found rodenticides in 70% 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, so this much larger dataset adds substantial weight to that earlier evidence. Across more than a decade of IERC research, the same underlying threat has persisted in multiple old-growth-dependent species, and it is now, for the first time, responding to policy.
The authors caution that the exact source of ongoing exposure is not fully resolved. State pesticide-use records only capture licensed commercial applicators, not household purchases or illegal use. Trespass cannabis cultivation, long documented by IERC as a major source of rodenticide contamination on public and tribal lands, remains untracked by any regulatory database, and legacy contamination from past illicit grow sites may also continue cycling through these forests well after active cultivation has stopped. Researchers say these hidden and historical sources likely explain at least part of the exposure that remains even as legal, regulated use declines.
About the Integral Ecology Research Center
Founded in Blue Lake, California, in 2004, IERC is a woman- and minority-founded nonprofit conservation science organization dedicated to conserving wildlife and the ecosystems they rely on across northwestern California and beyond. IERC partners with tribal, state, and federal agencies to conduct field research, from GPS telemetry and forensic wildlife science to toxicology and habitat monitoring, that translates directly into on-the-ground conservation action for species. Learn more at iercecology.org.
Publication: Jirinec, V., Gabriel, M.W., Peery, M.Z., Higley, J.M., Hofstadter, D.F., Varian, C.P., Rex, A., Franklin, A.B., Poppenga, R.H., Woods, L.W., Volker, S.F., Wengert, G.M. 2026. Forest food web contamination by rodenticides is pervasive but responsive to regulation: Insights from Barred Owl biomonitoring. Environmental Pollution, https://doi.org/10.1016/j.envpol.2026.129205
###
Environmental Pollution
Observational study
Animals
Forest food web contamination by rodenticides is pervasive but responsive to regulation: Insights from Barred Owl biomonitoring
26-Sep-2026