This past spring, birders across Chicagoland were devastated by the loss of a great horned owl family nesting in Lincoln Park. The three owls, an adult male, female, and their single offspring, gained much attention within Chicago’s birding community, with many flocking to North Pond to catch a glimpse of the small family unit.
Unfortunately, this enthusiasm ended in sorrow as the Chicago Bird Collision Monitors found the male owl unresponsive, later dying in mid-April due to what was then an unknown condition, followed soon after by the death of the female and the owlet - an entire family of owls, all dead in a span of only a few weeks.
Two of the great horned owls that lived in Lincoln Park before their deaths. Photo by Tess Branklin.
There was only one question that remained: what killed the three owls?
Some suspected that the owls may have collided with a window, a common and growing problem for migratory birds. Others feared that one of the owls' food sources, rats, might have consumed rodenticides, thus in turn poisoning the owls themselves. It’s common knowledge that Chicago, like many big cities, has a huge infestation of brown rats, the population of which has been growing exponentially, even beyond the city of Chicago to places like Evanston.
One of the attempts to curb the rat population is the use of poison bait - rodenticides. However, there’s a major flaw in this; the use of rodenticides has resulted in a spillover effect where other animals consume the poison and die. In turn, their bodies are eaten by birds of prey and other predators, who also prey upon the poisoned rats, resulting in their own deaths as well.
This scenario seemed plausible, given that this was the case with the recent death of Flaco, the Eurasian eagle-owl living freely in New York City for a year after escaping the Central Park Zoo. While Flaco suffered a traumatic injury when colliding with a window, traces of rodenticide, in addition to severe pigeon herpesvirus, were found in his system, all contributing to his death.
Therefore, it’s not unreasonable to assume that rodenticides also played a part in the deaths of the owl family of Lincoln Park. This conclusion was confirmed by Sarah Reich, head veterinarian at DuPage Wildlife Conservation Center in Glen Ellyn.
Flaco, the Eurasian eagle-owl of New York City, whose death was partially caused by rodenticides. Photo by Andrew Lichtenstein/Corbis.
“All three owls came back with multiple [anticoagulant, or] AC rodenticides in their bodies,” said Reich when asked by the Chicago Bird Alliance what the forensic results were. “Final cause of death for all three was listed as rodenticide toxicosis.”
Unfortunately, the demise of the owl family of Lincoln Park is just one of many examples of birds being killed by rodenticides, and DuPage Wildlife Conservation Center has been keeping track of the startling data gathered across Chicagoland.
“We have had 64 cases [confirmed] as ‘toxin-rodenticide’ since 2019,” said Reich. “That would include all rodenticides but the majority would be [AC].”
Reich also said that the number of birds affected has varied per year, with only six birds poisoned by rodenticides brought to the Center in 2021, before quickly jumping to 24 birds in 2022. Reich believes that the numbers may be even higher than that, due to birds dying before they arrive at the Center or before receiving treatment, having other comorbidities with rodenticide not being listed, or being subclinically or mildly affected, leading to AC rodenticides not being suspected.
The body of a red-tailed hawk found in Chicago, showing signs of rodenticide poisoning. The bodies of the owl family were found showing similar signs. Photo by Annette Prince.
“Reliable pre-mortem testing for AC rodenticides doesn’t exist currently,” said Reich. “So clinical signs, severe anemia, and slow to absent clotting is used to diagnose. If reliable testing does become available, it will be much easier to say whether numbers are going up or down since we could add it as a screening tool for more than just clinical animals.”
All of this paints a dark picture for birds, along with the rest of the environment. Not only are rodenticides not working to curb the brown rat population, but the poison’s spillover effect is harming other animals, including birds of prey. The CBA recently passed a policy calling for a reduction in the use of AC rodenticides, and a team is looking into solving this problem.
But what can residents of the Chicago area do to help protect birds from being poisoned? Is there any alternative to rodenticides to stop the overpopulating rats? Fortunately, Larry Heaney, Negaunee Curator of Mammals at the Field Museum, came up with an idea.
In a presentation for the Nurturing Nature series presented by Natural Habitat Evanston, Rotary International, Evanston Public Library, and the Evanston Ecology Center back in July, Heaney discussed the overpopulation of rats in Evanston, the ineffectiveness of rodenticides (along with other methods) to reduce the rat population and an alternative solution.
A brown rat looking for food on top of a garbage bin.
“The only effective way to control rats,” Heaney said, “is to control the food source.” The number of rats in a given population depends entirely on how much food a female can access. The more food, the more babies a female will have.
Heaney explained that brown rats’ primary food source is easily accessed trash in Evanston. If trash is made more secure and difficult for the rats to obtain, their population will decrease with smaller broods from the females.
If trash is made harder for rats to access, their population will naturally decline, leading to the discontinuation of rodenticides, thus ending the spillover effect. This can help prevent more birds of prey and other animals from getting poisoned.
The Lincoln Park owl family was beloved by birders across Chicagoland, and their deaths were heartbreaking. Sadly, we can’t change their fate, but if we can prevent our garbage from being accessible to rats and thus reduce the use of rodenticide, we can prevent other birds of prey from succumbing to the same outcome.
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