Eagles are majestic, beautiful, and full of toxins that we’ve pumped into the environment. A new study finds that 82% of dead eagles examined between 2014 and 2018 had detectable levels of rat poison in their systems.
The study, published on Wednesday in PLOS One, was, happily, made possible by the fact that eagle populations are actually rebounding across the U.S. The 303 birds examined in the study, a mix of bald and golden eagles, were collected by state and federal agencies that are part of the Southeastern Cooperative Wildlife Disease Study, a project based out of the University of Georgia that analyzes carcasses of wild animals from across the country for disease and other problems.
“When you have populations increasing, of course some are going to get into trouble, and that’s how they come across our shop here,” said Mark Ruder, an assistant professor at the College of Veterinary Medicine at the University of Georgia and a co-author of the study.
Ruder and his team saw an opportunity with the increased number of samples they were getting. “There had been a lot of attention on this type of rodenticide in other raptor species like barn owls, screech owls, hawks, but none specifically for eagles,” he said. “We knew anticoagulant rodenticide was a problem, so we had the anticipation that this is going to be worthy of an investigation.”
The study examined the eagle carcasses for evidence of what’s known as second-generation anticoagulant rodenticide, a heavy-duty solution for pests that acts as a blood thinner and eventually kills the animal after they eat it. Before the 1970s, anticoagulant rodenticides worked less well than current rodent killers. The rodents would often have to eat a lot more of the poison over several days to make it have an effect. Manufacturers began strengthening their products and introduced a second generation of the poison, which could kill rats and mice much quicker. This version of the rodenticide, however, is “stickier,” Ruder explained, meaning it stays in animals’ bodies—and can enter the bodies of predators that feed on rats and mice that may eat the poison.
“It’s the ability to persist in those tissues for a long time that creates the problem,” Ruder said. “Being efficient predators and scavengers, eagles are more at risk for accumulating this toxin through their system, basically just by being eagles—eating dead stuff or killing things and eating them.”
While 82% of the creatures in the study had measurable amounts of the poison in their bodies, it’s worth noting that anticoagulant rodenticide was determined to be the cause of death for only 4% of the sample of eagles examined. (The paper asserts that the mortality figure could be an underestimate, especially since around half the eagles in the sample died by trauma like gunshots, car accidents, or electrocution.) But the fact that such a high proportion of the animals had traces of rodenticide at all was “surprising,” Ruder said—and worrisome. Because of the danger posed to other animals, the Environmental Protection Agency has restricted the sale of anticoagulant rodenticide to just commercial markets and professional pest controls.
“If you go to the hardware store, you and I just can’t buy it willy-nilly,” Ruder said.
This control has had some good effects: Ruder said a study of barn owls in Kentucky had found that the birds’ level of exposure to anticoagulant rodenticide decreased after the EPA restricted use of the pesticide. Eagles, however, don’t generally prey directly on the main targets of the rodenticide like other raptor birds do, suggesting that the toxin got into their bodies another way.
“It’s not super common to look out the window and see a bald eagle hunting for the mouse that lives in your crawl space, or whatever,” Ruder said. “These are food webs—everything is connected—and the eagles are likely getting exposed to these compounds through ingestion of much more natural prey that have, in turn, eaten something else in order to be exposed.”
And this infection of the food web, the study suggests, is a widespread problem for wildlife across the country. The eagles came from 18 different wildlife management agencies from different regions, with no difference in levels of rodenticide between states.
“There is definitely recognition by many that these second-generation anticoagulant rodenticides have the potential to extend beyond their population of pest rodent species, and this study will certainly reinforce that understanding,” Ruder said. “Despite current management and policy efforts to mitigate that risk to wildlife, it still appears that we’re getting quite a bit of non-target exposure and intoxication in non-target species. We need to keep examining what pathways for exposure are for wildlife and figure out how to lower that risk.”