This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: 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. The study, published in the journal Environmental Pollution, analyzed liver tissue from 1,003 barred owls collected across northwestern California between 2019 and 2025.
It is 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 provides 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, 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 many as 46% of the owls sampled carried at least one rodenticide compound in their systems.
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 a research scientist at IERC. "Our owls were apparently healthy birds going about their lives.
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