Evidence of Rodenticides Found in California Owls

By M.R.Guercio

Anticoagulant rodenticides (AR) are poisons that are unfortunately used to kill rodents, such as rats and mice, and are widely available for use by farmers, homeowners, and large agribusiness alike. A paper published in Avian Conservation and Ecology examined the effect of ARs on populations of northern spotted owls and barred owls in remote forest lands of northern California. Many similar studies have focused on the presence of ARs in wildlife that live in urban or agricultural settings, where it was believed rodenticide use would be more prevalent. This study, though, found increasing amounts of ARs present in remote forest settings where it is hurting non-target forest carnivores. The route of exposure for such animals is by consuming a rodent who has been exposed to the poison. The study found that 40% of barred owls and 70% of northern spotted owls that they tested were exposed to at least one of eight ARs. Also, the use of what are called second generation ARs is prohibited in agricultural settings without human dwellings, yet there were owls collected in such areas that tested positive for secondary AR. While most of the ARs detected in the owls were at trace levels and not the cause of death, it is still alarming that this poison is being found in these remote areas and that secondary ARs are being used illegally. Even more alarming is that the northern spotted owl is listed under the Federal Endangered Species Act and listed as threated under the California Endangered Species Act, and is being further negatively impacted. There are plenty of non-lethal effects of ARs which include reduced clutch and brood size, fledgling success, slower blood clotting time, and residual AR transfer to eggs that lead to the suffering of many owls and other birds of prey.

Earlier studies concerning ARs have found extremely high concentrations in fishers, and have linked this exposure to thousands of illegal marijuana cultivation sites on public and tribal lands. Heading into the future as marijuana becomes legalized, this is something to be concerned about and warrants examination for a safer alternative to poisoning animals that are simply living and behaving as they naturally should. A creative alternative to rodent “problems” in agricultural settings is being implemented by California’s Hungry Owl Project, founded and directed by former wildlife rehabber Alex Godbe. In 2013 Godbe had groups of barn owls in 25 vineyards working to naturally reduce the rodent population that had been damaging their crops, without the use of rodenticides. This can be the case for other predators too such as coyotes, foxes, skunks, raccoons and opossums, to name a few. A good point made by Godbe is that we are killing off nature’s own rodent control by the second-hand poisoning of rodent eating predators (Williams).

While the use of pesticides such as ARs is a large issue that may sometimes seem out of our direct control, there are still things we can do in our daily lives that can either lessen or add to the existing problem. Pesticides have a wide-reaching effect that isn’t always in the forefront of a person’s mind when they decide to use them. Before you use Ars, keep in mind that you are killing not only mice that live in or around your home, but you are potentially poisoning many other animals and their offspring as well.  Friendly options are available that are safer for humans and animals such as catch and release mouse traps.  For more detail, please visit:

Gabriel, Mourad W., et al. “Exposure to Rodenticides in Northern Spotted and Barred Owls on Remote Forest Lands in Northwestern California: Evidence of Food Web Contamination.” Avian Conservation and Ecology, vol. 13, no. 1, 2018, doi:10.5751/ace-01134-130102.

Williams, Ted. “Poisons Used to Kill Rodents Have Safer Alternatives.” 


Marissa Guercio is a biology graduate from SUNY New Paltz who is interested in conservation and animal behavior.