GRAFTON – It has long been established that birds of prey have been ingesting poison from mice, rats and other small animals they catch, but the numbers may be on the increase, according to a study by Dr. Maureen Murray, a professor of wildlife medicine at Tufts University’s Cummings School of Veterinary Medicine.
Murray has been studying the impacts of rodenticide poisoning on hawks, owls and other birds of prey for more than a decade. In 2011 she published a paper showing the presence of anticoagulant rodenticides in 86% of birds tested over five years at Tufts. The study helped shape EPA safety standards that resulted in certain products being removed from the counters of home improvement and garden supply stores. Regulations that went into effect in 2011 were intended to prevent direct sales of ARs to the general public, limiting sales to rodent control professionals to use as needed.
A new study recently published by Murray in Environmental Toxicology and Chemistry magazine detected rat poison in all 43 red-tailed hawks tested by the Tufts clinic between 2017 and 2019. All the birds tested had been brought into the wildlife clinic to be treated for various reasons but had not survived due to injury or illness. Murray found evidence of ARs in all of the birds tested, and two or more types of ARs in 91% of the birds.
Murray said only a portion of the birds died directly from ingesting the poisons, but all had some amount of contamination from ARs.
In the study, Murray studied the livers of the dead birds to determine if they were exposed to toxins. She also took blood samples to see if blood could be used as a way of testing for ARs in living birds. She said that unfortunately the blood sampling did not prove to be a reliable method for testing for poisoning.
ARs kill rodents by thinning their blood, causing them to slowly bleed to death. Murray said that red-tailed hawks were chosen for the current study for several reasons. First, they had been part of two previous studies she conducted that also included three types of owls. From the previous studies, she said she knew hawks had been found to have high amounts of exposure to the rodent poisons. They also are the bird of prey most often seen at the clinic, whether from health issues including poisoning or injury from accidents. The third reason is they are generalists. They find food in a wide variety of species, giving an indication of how widespread the contamination is in the food chain.
Among the unknowns still to be studied are what specific species the hawks have gotten their contamination from. Smaller bird species have been found with poison in them as have other small mammals. Where the rodenticides come from also needs to be answered. The EPA regulations limit the use of ARs to professional pest control companies, but Murray said homeowners may be getting the poisons from other sources, or from a supply they had before the new regulations went into effect. They might also be able to locate them on the internet.
The poisons can also linger in the environment and in tissues, affecting wildlife long after they are used. Murray said the compounds have very long half-lives.
“These are big unanswered questions that will require a whole lot more research,” she said.
Murray said she plans to continue her research into impact of poisoning of wildlife. She said in part the research helps to measure the effectiveness of regulations that are designed to prevent the contamination. She said that her data set, which goes back to 2006, is invaluable in determining over time just how widespread the problem is and whether the situation is improving or getting worse. She said the research also helps draw attention to the problem.
“Part of the goal is to raise awareness of the risk of these rodenticides not only to birds of prey but other animals affected,” she said.
Murray said she hopes the research will cause people to pause and consider what methods they are using to control mouse and rat problems in their homes and businesses.
Among the suggestions Tufts offers include rodentproofing your home by plugging up holes where they can get in and securely containing potential food sources. Consider the use of snap traps and opt for products that comply with EPA regulations.