Toxic Threads: The Unintended Repercussions of Chemical Contaminants
When Flaco, a Eurasian eagle owl, escaped from his enclosure in the Central Park Zoo, experts put his chances of survival in the urban ‘wild’ to be quite low, with his limited hunting skills. But he surprised everyone, becoming a fixture in Central Park, hunting New York City’s rampant rat population. Tragically, a little over a year after his escape, he died in February 2024. Onlookers had seen him fly into a building, and his death was attributed to acute trauma from the collision.
While glass facades of buildings pose a definite threat to urban birds, they are far from the only struggle many birds of prey face due to humans. An autopsy on Flaco later revealed dangerously high levels of four different anticoagulant rodenticides in his system. These rodenticides work by thinning blood and preventing clotting, leading to disorientation, internal bleeding, organ failure and ultimately, death. It is possible that this induced disorientation led to Flaco’s fatal collision, in addition to making him unwell. While the federal government banned the retail sale of these rodenticides in 2015, they remain legal for use by pest-control agencies. When rodents eat these and don’t immediately die, they become sluggish and therefore easy prey for urban raptors. As raptors ingest several of these rodenticide ridden rats, the levels of these chemicals keep increasing in their bodies. This phenomenon, called biomagnification, is prevalent across the animal kingdom — pollutants are not broken down in bodies and accumulate, and their levels increase as they move up the food chain.
Flaco’s death is one among many chemical poisoning deaths that conservationists and wildlife managers see among apex predator birds. A resident great horned owl, a favorite among local birders including myself, died in November 2024 in a Massachusetts Audubon Sanctuary due to rodenticide accumulation. In early 2024, a family of great horned owls in Chicago faced a similar fate, as have many other raptors over the last few years, including hawks, falcons, eagles and barred owls. A global review found that across various raptor species, nearly 60% of tested individuals showed residues of these rodenticides in their systems .
The bioaccumulation of anticoagulant rodenticides is reminiscent of other chemical pollutants that have harmed wildlife. DDT, a previously commonly used pesticide, was found to cause the thinning of bird’s eggshells, leading to population declines in the bald eagle and the peregrine falcon, among others, before its ban in 1972.
Feathers can serve as vital indicators of the toxins that might be accumulating in these birds’ bodies. Feathers from the albatross are used to study increasing levels of mercury, a potent neurotoxin. A novel study using museum specimens revealed concerning levels of mercury biomagnification — modern albatross feathers had double the amount of mercury than feathers from 120-year old museum albatross specimens.
Recently, another class of chemicals that are hard to break down, called PFASs, have been accumulating in the environment, including in feathers of birds and in rainwater. PFASs are used ubiquitously, in household applications such as cookware to industrial lubricants and firefighting foams. Biomagnification of these chemicals can lead to developmental issues and cancer in fishes, birds, and mammals, including human beings.
Chemical pollution remains a major problem, especially since a vast number of chemicals don’t degrade, and their effects on wildlife and humans is relatively understudied. We are synthesizing new chemicals at an alarming rate, with limited regulation of their use. While it is crucial that we test more chemicals that we are pumping into the ecosystem, this seems to be getting more and more challenging.
With rodenticides, however, safer alternatives do exist. Raptors that are harmed by rodenticides can themselves be a way to control rats. The Hungry Owl Project encourages homeowners to install nest boxes for barn owls, which are natural predators of rodents and can help control their population; a single family of barn owls can eat up to 3400 rodents a year!
Another alternative is on the horizon — rat birth control. New York City recently passed Flaco’s Law, named in honor of the late eagle-owl that captured the hearts of so many. This will launch a pilot program of using rat contraceptives in the city. These contraceptive pellets, disguised as tasty treats, target the ovarian function of female rats and sperm production in male rats. Flaco’s year-long adventure as a free owl in Central Park ended in tragedy, but his legacy will be to change the fate of countless other birds.