High rates of anticoagulant rodenticide exposure in California Barred Owls are associated with the wildland–urban interface

Pesticide use is pervasive and the exposure of non-target wildlife has been well documented over the past half-century. Among pesticides, anticoagulant rodenticides (AR) have emerged as a particularly important threat in forests of the western United States, with exposure and mortality reported for several species of conservation concern. To further quantify this threat, we collected specimens of Barred Owls (Strix varia) and Barred Owl x Spotted Owl hybrids from the Klamath and Cascade Mountains and Sierra Nevada in California, USA to use as indicator species for environmental contamination with AR and to infer exposure of closely related and ecologically similar Northern and California Spotted Owls (S. occidentalis caurina, and S. o. occidentalis, respectively). We tested 115 Barred Owl and 12 Barred Owl x Spotted Owl hybrid livers for 8 AR compounds and found high rates of exposure (62%) across our study area, and greater than previous studies in the Pacific Northwest. In addition, we sampled 7 ovaries from 7 females and 100% tested positive for AR. Female Barred Owls were more likely than males to be exposed (78% and 50%, respectively). Unlike previous studies, we found no clear link between illegal cannabis cultivation and AR exposure. However, Barred Owls sampled in proximity to the wildland–urban interface (WUI) were more likely to be exposed to AR. Though the exact source (e.g., cannabis cultivation or application around human dwellings) and location are unknown, the association of AR exposure with the WUI was supported from GPS data from Barred Owls, Northern and California Spotted Owls, and hybrids using the WUI for foraging. The high rate of AR exposure in Barred Owls and hybrids provides mounting evidence of an additional stressor that ARs may pose to Spotted Owls—including the first evidence for California Spotted Owls—and fauna native to western forest ecosystems. LAY SUMMARY Anticoagulant rodenticides have emerged as an important threat in forests of the western United States, and it is vital to understand how and where wildlife is exposed. As indicator species for Spotted Owl exposure, we screened 115 Barred Owls and 12 Barred Owl x Spotted Owl hybrids, collected from northern California, USA for 8 anticoagulant rodenticides. 62% of owl specimens (72 Barred and 7 hybrid) were exposed to anticoagulant rodenticides, in particular to the acutely toxic, second-generation class. Females and owls sampled close to the wildland–urban interface were more likely to be exposed to anticoagulant rodenticides. GPS-tagged Barred and Spotted Owls commonly foraged in the wildland–urban interface, suggesting Spotted Owls are also likely at risk of exposure. The high rate of AR exposure in Barred Owls and hybrids provides mounting evidence of an additional threat to Spotted Owls.