Forest food web contamination by rodenticides is pervasive but responsive to regulation: Insights from Barred Owl biomonitoring.

Anticoagulant rodenticides and their substitutes are widely deployed in urban and agricultural settings, but evidence is mounting that contamination extends into remote forests through diffuse sources. Most wildlife exposure data come from opportunistic carcass collections that systematically overrepresent animals near human activity and in compromised condition. We used experimental removals of Barred Owls (Strix varia) from non-urban forests of northwestern California as a minimally biased sentinel sample, analyzing liver tissue from 1,003 individuals collected between 2019 and 2025 for 14 rodenticide analytes. In the regional dataset (n = 837), anticoagulants were detected in 35.7% of owls (second-generation 35.2%, first-generation 1.2%). Brodifacoum was most prevalent (30.6%), followed by bromadiolone (14.2%). Cholecalciferol, reported separately because it also occurs endogenously and was not screened in previous Strix owl surveys, was detected in 18.5% of individuals; 9.9% of owls carried cholecalciferol but no anticoagulant. Bromethalin was not detected, though liver is a suboptimal tissue for this neurotoxicant. Exposure rates, concentrations, and compound richness were higher in females than males and increased across age classes, consistent with bioaccumulation of persistent compounds. In subadult subsample from Hoopa Valley (n = 292), model-predicted anticoagulant exposure fell from ∼50% in 2019 to ∼20% in 2024, coincident with tightening California restrictions on anticoagulants-a pattern consistent with a measurable regulatory effect on wildlife exposure. Concurrent increases in cholecalciferol detections indicate possible product substitution that warrants monitoring. That restrictions on the most persistent anticoagulants produced a detectable signal within years demonstrates that policy can translate into measurable wildlife benefit.