Rodenticides and Snowy Owls: A Growing Problem

Scott WeidensaulUpdates5 Comments

A memorial for Flaco, the Eurasian eagle-owl. (Rhododendrites CC BY-SA 4.0)

The death in New York City in late February of Flaco the Eurasian eagle-owl made news all over the world; many people had been rooting for this captive-bred male, who escaped from the Central Park Zoo in a year earlier after someone vandalized his enclosure, and had been living free in the Big Apple ever since.

At the time of his death, it was assumed Flaco succumbed from flying into a window, but the results of a necropsy conducted by the Bronx Zoo made clear that Flaco was already quite ill, and suffered from two major health problems: a disease known as pigeon herpesvirus that he contracted from eating feral pigeons, and significant levels of anticoagulant rat poisons, either of which could have led to his death.

Flaco was a celebrity owl, and his demise drew a great deal of attention to the threat that rodenticides pose to raptors of many sorts — including snowy owls. Since Project SNOWstorm’s inception in 2013, our team of wildlife veterinarians have been performing similar necropsies and toxicology tests on hundreds of snowy owls that have been found dead or died while in rehabilitation. We’re in the process of finalizing a massive analysis of this snowy owl health and disease database, the largest of its kind that we’re aware of anywhere in the world, which we’ve undertaken in collaboration with Dr. Nicolas Lecomte’s team at Moncton University in New Brunswick, Canada. As with everything we do, this work was made possible by donations to Project SNOWstorm from the general public, for which we are deeply grateful.

Given the attention Flaco’s death has brought to the issue of rodenticides and raptors, we asked Cassandra Cameron, who is spearheading the analysis, to update SNOWstorm’s community about the very troubling results she’s found regarding the rapidly increasing threat these powerful toxins pose to snowy owls — and all raptors. Please understand that these results are preliminary, and the details may change to some degree as we complete final statistical analyses and submit our findings for peer-reviewed publication. But in the meantime, here is Cassandra’s report:


Anticoagulant rodenticides are used globally to control undesirable rodent populations in urban and agricultural environments. Common active ingredients include brodifacoum, bromadiolone, diphacinone, difethialone, dicoumarol and chlorophacinone. They act by preventing the blood from clotting normally, which causes the animal that has ingested it to bleed to death. Although intended to target rodents, these toxins often enter the food web, exposing predators such as eagles, hawks and owls to secondary poisoning when they prey on disoriented or lethargic rodents that have ingested rodenticides. This poses a significant threat to the raptors themselves.

Since 2013, we have assessed anticoagulant levels in 196 snowy owls that had previously died of various causes. Of these birds, 35 percent showed quantifiable amounts of anticoagulants (more than a trace amount). Evaluating safe levels of these highly potent toxins in raptors is challenging due to variations in tolerance caused by factors such as sex, size, age and the condition of the birds. However, it has been suggested that levels over 0.03 ppm could lead to mortality in raptors.

In our analyses, 44 birds (22 percent) were over that 0.03 ppm threshold. Ninety-three percent of them had signs of internal hemorrhaging and bleeding. While there are other common causes of hemorrhage in snowy owls, such as vehicle strikes, many of these birds showed no other signs of injuries, such as wounds or fractures. And even for the birds that suffered obvious trauma, it is possible that anticoagulant poisoning, which reduces the blood’s ability to clot, made them more vulnerable to injuries in the first place.

This graph shows the alarming rise since 2013 in the number of snowy owls showing rodenticide levels above the threshold believed to lead to death. (©Project SNOWstorm)

Perhaps most alarming, our preliminary results seem to indicate an upward trend in snowy owls’ level of exposure. While the number of tested birds over the threshold was near zero about 10 years ago, by 2022, 56 percent of those tested were over the suggested threshold of toxicity — far above the 22 percent average for the entire 10-year period.

Ironically, our necropsy data also show that a lot of snowy owls had rodents in their stomachs when they died, including the invasive Norway rat. Therefore, they might have been killed just as they were offering us an invaluable ecosystem service by getting rid of the very rodents the poisoned baits were set for.

To protect our raptors of all species, it is imperative that alternatives to poison bait should be considered. For example, by encouraging more natural predators through habitat management (e.g. installation of hunting perches, nesting boxes, and undisturbed habitats), by using conventional snap traps, or by reducing rodents’ access to trash and pet food, we may be able to control rodent populations while protecting their natural predators.


Although Project SNOWstorm is best known for tracking the movements of snowy owls using GPS telemetry, the work of our veterinary team, in collaboration with colleagues across the U.S. and Canada, is shining a critical light on the threat environmental contaminants pose to snowies and other raptors. Rodenticides are, sadly, just one group of toxins that show up in the snowy owls we necropsy; others include methyl mercury, heavy metals like lead, cadmium and selenium, and long-lasting organic compounds like DDT and DDE. Once Cassandra’s sweeping analysis of this unprecedented dataset is completed, it will give the first full picture of the degree to which these and other contaminants are threatening snowy owls. We will share those results with you all as well.

None of this work would be possible without the support the public has shown to Project SNOWstorm over the past decade. Thank you.

And Then There Were...None?

5 Comments on “Rodenticides and Snowy Owls: A Growing Problem”

  1. Thank you for this. Windows are bad enough for many world species, but more than brodifacoum needs to be outlawed in the US. I think it was ~2006, when Israeli and Palestinian farmers replaced their rodenticides together with Common Kestrel and Barn Owl nest boxes, with a huge success. Old and new rodenticides must be made illegal and replaced by the birds themselves here. I may be naive, but eventually together I believe it can be done after an uphill battle. Rodenticides are cruel!

  2. There is a simple solution to this. The bait stations placed on the outside of buildings should be one-way traps, and if the mice need to be killed, it should be done humanely.

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