Cities are at war with rats. Flaco the owl and other animals pay the price.

Flaco the owl had lethal levels of rat poison in his system. Rodenticides often poison birds of prey as well as mountain lions, bobcats, and fishers.

• Flaco, the loose Central Park Zoo owl, died with lethal levels of rat poison  in his system.
• Rodenticides used in cities like New York also poison other wildlife, especially birds of prey.
• The problem could become worse as cities ramp up their efforts to combat rats.

Flaco, the owl who escaped from the Central Park Zoo last year and transitioned to life as a free New Yorker, had lethal levels of rat poison in his system when he died last month — a problem that impacts non-famous birds of prey and other animals as well.

The Eurasian eagle-owl got loose in February 2023; the mesh of his enclosure was found tampered with. Police have said the investigation is ongoing, but no information about potential suspects has been released. After over a year flying wild, Flaco collided with a building on the Upper West Side and died on February 23.

The Central Park Zoo said in a statement Monday the necropsy showed Flaco had two underlying conditions: severe pigeon virus, from preying on Manhattan pigeons, and exposure to four different kinds of rat poison that the city employs to combat its rodent problem.

"These factors would have been debilitating and ultimately fatal, even without a traumatic injury, and may have predisposed him to flying into or falling from the building," the zoo said.

The zoo noted the contributing factors that caused Flaco's early death "underscore the hazards faced by wild birds, especially in an urban setting."

Indeed, many birds and other wildlife are regularly poisoned by rodenticides that are meant for rats — an occurrence that could become increasingly likely as cities like New York, Chicago, and Boston up the ante on their rat-fighting efforts.

Anticoagulant rodenticides — which prevent blood from clotting and cause animals to bleed to death internally — used to target rats have inadvertently poisoned other species worldwide, with birds of prey being most impacted.

A 2018 literature review published in the Journal of Veterinary Medical Science found rat poison was detected in 60% of various raptor species globally, which can include owls, falcons, hawks, and eagles.

The birds can be exposed to rat poison in several ways: by directly coming into contact with it or by eating poisoned prey, which could be rats or mice, or even a non-rodent species, like small mammals and reptiles, that were also unintentionally exposed to it. Because the rat poison does not kill the animal for several days, there's time for an owl to prey on it and also injest the poison. Even once the animals are dead, they still maintain levels of poison that can transfer to scavengers.

A 2020 study authored by Maureen Murray, director of Tufts Wildlife Clinic in Massachusetts, found that 100% of the 43 red-tailed hawks sampled at the clinic between 2017 and 2019 had been exposed to rat poison. Murray told Tufts Now that the numbers of raptors with rat poison seen by the clinic had steadily increased.

"The ability of these rodenticides to permeate the food chain and ecosystems is pretty remarkable," Murray said, adding, "Red-tailed hawks eat a lot of small mammals, but they also eat birds, reptiles, or amphibians; they might scavenge. Ultimately, their prey base is very contaminated."

Another study from 2018 conducted in California found 70% of northern spotted owls, a protected threatened species under the Endangered Species Act, tested positive for rat poison, while 40% of barred owls tested positive.

But the poisoning is not just an issue for birds of prey. A 2018 analysis by the California Department of Pesticide Regulation found 85% of mountain lions, bobcats, and fishers that were tested had been poisoned by rodenticides. Raccoons, foxes, skunks, coyotes, and house pets can also be exposed to rat poison, according to the California Department of Fish and Wildlife.

While some cities are amping up the use of rat poison, others are addressing the threat posed to wildlife. The use of rat poison has been restricted in California for years, though the rodenticides continued to show up in animals that were not being targeted.

In October, Gov. Gavin Newsom signed a bill expanding the moratorium on certain rat poisons.

"We really needed these protections to ease the needless suffering of pumas, foxes and owls," Tiffany Yap, a senior scientist at the Center for Biological Diversity, said in a press release, adding: "Rat poison indiscriminately harms animals up and down the food chain."