Liver tests on 1,003 barred owls trace falling anticoagulant exposure and a rising substitute poison

This study is worth reading closely because of where its owls came from. Most rodenticide surveys start from carcasses and sick animals. "Nearly all rodenticide surveys rely on animals that turned up sick or dead, so they tend to capture the worst cases," said Vitek Jirinec, the study's lead author and a research scientist at IERC ^[11]. The 1,003 owls here were collected across northwestern California from 2019 to 2025 ^[1] and removed under an ongoing regional conservation program ^[3], so the denominator is the living population. "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," Jirinec said ^[12]. IERC describes the collection as the largest rodenticide biomonitoring dataset assembled for a single wildlife species in North America ^[2].

The headline decline covers fewer birds than it first appears to. The fall from about 50% to about 20% applies to young owls ^[4]. That is a relative drop of about 60% ^[1]. Young birds are a sensible group to track. Older adults and females carried the most exposure and the widest mix of compounds, a pattern the authors link to accumulation over a lifetime ^[9], so a juvenile's liver is closer to a record of recent exposure.

I think the laws are the leading explanation for the drop, though the evidence is a timing match in one region. According to the study, the decline lines up closely with AB 1788, the California Ecosystems Protection Act passed in 2020, and with AB 2552, a 2024 law extending restrictions to older-generation compounds ^[5]. The young-owl series ends in 2024 ^[4], so most of the change falls under the earlier law. Illegal use is also part of the history. "We've watched illegal use and unregulated products drive rodenticide exposure in these forests for more than a decade," said Greta Wengert, IERC's co-director and the study's principal investigator ^[13]. A 2012 IERC study tied anticoagulant exposure in Pacific fishers to trespass cannabis cultivation on public lands ^[15]. The thing this doesn't tell you is how much of the decline came from restricted sales and how much from changes in illegal use. Wengert is firmer. "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," she said ^[14].

Cholecalciferol, vitamin D3 sold as a rodenticide, was screened for in this species for the first time and turned up in 18.5% of the owls ^[6], roughly 186 birds ^[2]. Its detections rose over the same years that anticoagulant exposure fell, a pattern the study says suggests product substitution as older compounds become harder to get ^[7]. The substitution reading rests on two trends moving in opposite directions, and I find it plausible. Counting every compound, as many as 46% of the owls carried at least one rodenticide ^[8]. That figure covers a different group from the young-owl 20%, so subtracting one from the other does not measure the substitution.

The heaviest loads sat in breeding-age females and established territory holders, the individuals a population depends on most to sustain itself ^[9]. Northern spotted owls are close relatives with a higher proportion of rodents in their diets, and the authors say the pattern may indicate added risk for spotted owl recovery ^[10]. A 2018 IERC study found rodenticides in 70% of sampled northern spotted owls in the same region ^[16].