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Life in the city can be dangerous for these birds of prey
New research shows that rat poison spreads farther through the ecosystem than they are intended to.
For as long as humans have stored food and built settlements, rats and mice have followed close behind.
Our attempts to control these rodents can affect other animals. And the effects can reach far higher up the food chain than previously thought.
“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 80 years.
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 end up in other species.
Efficiency comes at a cost
“I have examined how birds of prey become exposed to rodenticides,” says Hindmarch.
She has specifically studied a type of rat poison that works by preventing blood from clotting. These toxins are used against both rats and mice.
Hindmarch has also worked to improve the methods used to detect this type of poisoning in birds of prey.
To do this, she studied owl diets and movements.
She has tested birds of prey for exposure and analysed hundreds of liver samples and post-mortem records from across North America.
The results 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 birds of prey most likely to be exposed to rodenticides.
Across three resident owl species in British Columbia, the amount of rats in their diet increased with the degree of urban development. The more urbanised the birds' habitats were, the more rats they ate.
At the same time, the results showed that rats are not the only route of exposure. Other prey species, such as small birds and small mammals, can also carry traces of rat poison in their bodies.
“This might explain why some bird-eating raptors were exposed at levels similar to those seen in rodent-eating species,” the researcher says.
Hindmarch’s findings show that owls that prey on rats in cities are particularly at risk. They also suggest that birds of prey are more sensitive to rat poison than previously assumed.
New rodenticides remain in the body longer
Most of the contamination Hindmarch found came from so-called second-generation anticoagulant rodenticides.
These substances were developed because rats and mice gradually developed resistance to older types of rat poison.
But these newer poisons are more toxic than the first generation of rat poisons. They also remain in the bodies of animals for much longer than older products.
This combination makes them particularly likely to persist in food webs and reach predators that were never meant to be exposed.
Difficult to detect
Hindmarch’s findings show how rodenticides can move through the food web and become widespread in birds of prey.
She points to a major challenge for wildlife conservation and the rehabilitation of injured birds: exposure to rat poison may not always be obvious.
The effects can be masked by other threats that are common in human-dominated landscapes. These may include vehicle collisions, electrocution, starvation, and falls from nests.
The results further suggest that birds of prey may be more sensitive to these poisons than previously believed, making it even more important to identify exposure early and accurately.
The human population continues to grow, and so is urbanisation. There are few good alternatives to current pest-control methodds. The need to control rats and mice is unlikely to diminish anytime soon.
“As long as people live in urbanised 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.”
Reference:
Hindmarch, S. Factors driving anticoagulant rodenticide exposure in raptors, Doctoral thesis at the Norwegian University of Life Sciences (NMBU), 2026.
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