When owls go to town, rat poison follows

For as long as humans have stored food and built settlements, rats and mice have followed close behind.

Our attempts to control rodents may be affecting predators far higher up the food chain than previously suspected.

“Rodenticides are among the most widely used tools for controlling rats and mice worldwide,” says researcher Sofi Hindmarch.

They are popular for a reason. Rodenticides are cheap, effective and easy to use. Unlike traps, they can control large rodent infestations with relatively little effort. They have been in use for almost eight decades.

Rodenticides may be aimed at rats and mice, but their journey often does not end there. As contaminated animals are eaten by predators, the poisons can move up the food chain and into wildlife.

Efficiency comes at a cost

Hindmarch is defending her doctoral thesis on raptors and pesticides at the Norwegian University of Life Sciences (NMBU).

“I have examined how birds of prey become exposed to rodenticides, particularly anticoagulant rodenticides,” she explains.

Anticoagulant rodenticides, often abbreviated to ARs, are poisons used to control rats and mice. They work by preventing blood from clotting and are widely used around the world. Hindmarch has also worked to improve methods for detecting AR poisoning in these birds.

To achieve this, she studied owl diets and movements, tested raptors for exposure, and analyzed hundreds of liver samples and post-mortem records from across North America.

What she found suggests that life in the city comes at a price for birds of prey.

More sensitive than previously thought

Hindmarch found that owls living in urban areas and hunting rats were the raptors most likely to be exposed to rodenticides. Across three resident owl species in British Columbia, rat consumption increased as the amount of urban development within their home ranges increased.

At the same time, the results showed that rats are not the only route of exposure. Other prey species, such as songbirds and small mammals, can also carry rodenticide residues.

“This might explain why some bird-eating raptors were exposed at levels similar to those seen in rodent-eating species,” she says.

Hindmarch’s results show that owls preying on rats and living in urban environments were the raptors most likely to be exposed. Further, raptors are more sensitive to rodenticide poisoning than previously assumed.

More toxic than the first generation

Most of the contamination found in Hindmarch’s studies involved so-called second-generation anticoagulant rodenticides. These compounds were developed to overcome resistance in rodents, but they are also more toxic and remain in animal bodies for much longer than older products.

That combination makes them particularly likely to persist in food webs and reach predators that were never meant to be exposed.

Masking the effects

Hindmarch’s findings show how rodenticides can move through food webs and become widespread in birds of prey. They also highlight a major challenge for conservation and wildlife rehabilitation: exposure may not always be obvious, and the effects can be masked by other threats common in human-dominated landscapes, such as vehicle collisions, electrocution, starvation and falls from nests.

The results further suggest that raptors may be more sensitive to these poisons than previously believed, making it even more important to identify exposure early and accurately.

As the human population is increasing so is urbanization. Given the lack of alternatives our dependency on rodenticides to control rats and mice is not going to diminish anytime soon.

“As long as people live in urbanized societies, storing food and producing garbage, there will be a need for rodent control,” Hindmarch says.

“The question is how we can use them more responsibly while reducing unintended impacts on birds of prey and other wildlife,” she concludes.