This young barred owl is typical of birds of prey at risk of ingesting anticoagulant rodenticides used to kill nuisance rodents, like mice and rats. Justine Kibbe Photo

Scientists agree that there is no such thing as a safe poison. That unfortunate truth applies to anticoagulant rodenticides (AR), which have been used for decades to kill nuisance rodents like mice and rats.

Rodenticides are anti-coagulants placed in bait stations to attract mice and rats. After feeding, rodents die from internal bleeding, but not immediately. While still alive, they are a food source for raptors, and after death, for scavengers. Ingestion transfers the poison to the birds.

A 2020 Tufts Wildlife Clinic study reported that 100 percent of the red-tailed hawks in the study tested positive for exposure to anticoagulant rodenticides. For the study, Clinic Director Maureen Murray, a wildlife veterinarian, sampled 43 red-tailed hawks, which were admitted to the clinic but did not survive due to their injury or illness.

Ms. Murray focused on these hawks, because they are most commonly seen at the clinic and are generalist predators, which offered a sense of how widespread the contamination is in the food chain.

“The ability of these rodenticides to permeate the food chain and ecosystems is pretty remarkable,” said Ms. Murray. “Other studies have shown residues in songbirds and insects. And that’s what this study reflects. Red-tailed hawks eat a lot of small mammals, but they also eat birds, reptiles, or amphibians that they might scavenge. Ultimately, their prey base is very contaminated.”

First generation ARs, chlorpophacinone, diphacinone and warfarin, were followed in the 1970s by a second generation of more toxic anticoagulant rodenticides (SGAR), brodifacoum, bromadiolone, difenacoum, difethialone. This study found that SGARs were more prevalent in the hawks than ARs.

In 2008, the U.S. Environmental Protection Agency (EPA) began tightening rules regarding sales and use of SGARs. This study’s findings are meant to measure the effectiveness of the EPA’s approach to regulating SGARs in preventing exposure of wildlife species.

Ms. Murray encouraged anyone looking for pest control solutions to consider approaches other than ARs: Find out where the mice or rats are coming in, plug up holes in the house or around windows, take away food and water sources, and clean out nesting sites.