Poison meant for rats or mice has claimed another unintended avian victim, this time an Arctic snowy owl spending the winter in the Chicago region.

The owl is recuperating at Willowbrook Wildlife Center in DuPage County, having been rescued Jan. 31 by volunteers with the Chicago Bird Collision Monitors after being found grounded with bloody legs and feet. 

Like a bald eagle brought to Willowbrook earlier in January, the owl had a small wound that bled profusely, due to a reaction to anticoagulant rodenticide poisoning, according to the wildlife center.

“We can’t emphasize enough that anticoagulant rodenticides are extremely harmful to our wildlife populations, especially raptors,” the center said.

Rodenticides mainly target rats and mice but also kill squirrels, chipmunks, woodchucks, voles, porcupine, beaver and nutria. These pesticides have different active ingredients and work in a variety of ways, but it's the potent anticoagulant formulas, typically containing brodifacoum or bromadiolone, that are particularly hazardous to raptors.

The poisons work by inhibiting an enzyme that helps recycle vitamin K, which is necessary for clotting. No enzyme means a body’s vitamin K will eventually run out; no vitamin K means no clotting.

Any tiny bump will cause a hemorrhage in an animal that’s ingested the poison, Dr. Sarah Reich, Willowbrook’s head veterinarian, previously told WTTW News.

Birds, it should be noted, don’t eat rat poison, Reich clarified. But they do eat the rodents that eat the bait. The anticoagulant rodenticides are so powerful and linger in an animal for so long, they get passed up the food chain.

The owl — likely a female or juvenile male, based on its brown-flecked plumage — is responding well to vitamin K therapy, the center said. “We can only hope to get this beautiful owl back out in the wild and the world it is so well adapted for.”

Contact Patty Wetli: @pattywetli | (773) 509-5623 |  [email protected]