Quantifying the frequency and effects of secondary exposure to rodenticides in barn owls

View the project final report

Commodities

Practices

• Crop Production: application rate management
• Education and Training: farmer to farmer, on-farm/ranch research
• Natural Resources/Environment: wildlife
• Pest Management: biological control, chemical control, integrated pest management
• Production Systems: agroecosystems

Barn owls are a popular component of Integrated Pest Management (IPM) programs for the control of rodent pests across the Western United States and contribute to sustainable agriculture through integration of natural biological control, enhancing the environmental quality of agricultural regions, and sustaining the economic viability of agricultural operations. However, because farmers utilize rodenticides to control rodent pests, owls can suffer from both lethal and sublethal secondary poisoning. Despite the important role that owls can play in providing long-term, sustainable, and natural pest control services, we have little understanding of how often owls are exposed to rodenticides and what effect this exposure has on their behavior and reproductive success.

To tackle this critical gap in knowledge, our study addressed five key objectives: 1) Determine if rodenticide exposure affects growth rates in owl nestlings; 2) Understand how land-use type, rodenticide applications, and prey choice affect the frequency of rodenticide exposure; 3) Inform predictive models on the efficacy of barn owls for controlling rodent pests on farms; 4) Create stakeholder-verified recommendations for the use of rodenticides in combination with barn owls for effective IPM; and, 5) Disseminate findings to producers through publications, a field-demonstration, visits to schools, and presentations to pest-control and agricultural groups.

From 2018 to 2021, we banded 224 nestlings and 67 adults. From 2018 through 2020, 44 nest boxes were monitored revealing a 79.5% success rate of at least one chick fledging from a nest box. There was low field application of rodenticides at our study sites and blood sampling did not always align with timing or location that baiting that occurred. We screened 231 breeding barn owls (adults and nestlings between February and August 2018 to 2021) from 121 nests across three counties for circulating rodenticide exposure and found one positive result for Chlorophacinone cted at trace (<5ppb) levels.

Our morphometric monitoring of nestlings (561 measurements from the successfully fledged nestlings and 273 measurements from failed nestlings) revealed that the average fledge age for a barn owl nestling was 55 days, or 7.8 weeks.

We dissected 1,595 pellets between 2018 and 2020 to identify the diet of barn owls living in agroecosystems. Rodent pests for each habitat made up over 95% of the barn owls’ diet. In vineyard systems, the preferred prey choices were mice and vole, whereas gophers were a more opportunistic choice. Video monitoring took place in 2020 and 2021. Seven nest boxes were used with a total of 38 nights recorded. We were able to use this average to estimate the amount of prey items consumed by nestlings when in the nest box. The average biomass removed during an eight-week period averaged to approximately 30 kg per nest box.

Between the years 2019-2021, we tracked 12 breeding female barn owls using Alle-300 Ecotone GPS units. We obtained an average of 2,929 GPS data points per owl, resulting in an average of 6.5 nights of movement tracking per owl. Qualitative assessments of our owl movement data confirm that barn owls breeding within California agriculture exhibit hunting behaviors in various crop fields, adjacent grassland habitats, along roads and highways, and around crop fields they nest in.

Overall, our results indicate that barn owls in this system foraged in habitat immediately surrounding nest structures as well as nearby agricultural and grassland habitats. In these habitats, the barn owls targeted rodent pest species and removed substantial biomass of prey species from the surrounding habitats. We found limited evidence of acute anticoagulant rodenticide in blood and pellet samples collected at our study site which corresponds with information gathered from collaborating producers who reported limited baiting. Together, these results highlight the utility of barn owls as an effective tool in integrated pest management strategies at both local scale and at a regional scale.

Future research should be focused on sampling for rodenticide exposure simultaneously with field rodenticide applications and in landscapes with a larger number of buildings and facilities where second-generation anticoagulant rodenticides are regularly used.

1) Determine if rodenticide exposure affects growth rates in owl nestlings;
2) Understand how land use type, rodenticide applications, and prey choice affect
the frequency of rodenticide exposure;
3) Inform predictive models on the efficacy of barn owls for controlling rodent pests
on farms;
4) Create stakeholder-verified recommendations for the use of rodenticides in
combination with barn owls for effective IPM; and,
5) Disseminate findings to producers through publications, a field-demonstration,
visits to rural schools, and presentations to pest-control and agricultural groups.