Some consider them pests or harbingers of doom. For others, they’re a sign of good luck. For Pullman resident Phoebe Murtagh, the American Crow is a charismatic addition to her favorite Chicago parks.

“I’ve found them curious since I learned that they can use tools and recognize faces, and clever animals are always kind of the most interesting because you end up wondering what they’re thinking about,” she said.

The 26-year-old spends a lot of time in parks all around the city and loves watching the large, black birds explore the tree tops. But the life-long Chicago resident says she doesn’t remember seeing many crows growing up.

“In the past five [or] 10 years, I see [crows] a lot more than I can ever recall,” she said. So she asked Curious City if the number of crows in the city has actually increased.

And it turns out Phoebe’s observation is correct. There are more crows in Illinois today than there were in the early 2000s when she was a child. Illinois’ crow population has gone through drastic changes in the last 20 years and the reason why isn’t necessarily what you might expect.

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It all goes back to 2001, when scientists discovered two dead crows in Chicago that tested positive for the West Nile virus.

Although most people only suffer mild illness, West Nile can be deadly. Since it was first detected in the U.S., more than seven million people have been affected by the virus, which also decimated the Illinois crow population.

The crow population still has not recovered to what it was in the 1990s. But by continuing to track the state’s birds, scientists have been able to learn more about how West Nile virus spreads and how to contain it. And changes in the bird population can serve as an early warning system for all kinds of viruses, not just West Nile.

West Nile devastates Illinois’ crows

In the late 1990s and early 2000s, Illinois was home to the largest gathering of American Crows in the world.

Every year, more than 300,000 crows would come to Danville, Illinois from around the Midwest to make their winter roost near Lake Vermilion.

And they weren’t just in central Illinois.

“[Crows are] one of those generalist species that can do pretty good anywhere,” said Tara Beveroth, an avian ecologist at the Illinois Natural History Survey. who was hired in 2004 to help figure out why so many birds were dying.

Crows are omnivores, so they do really well in cities like Chicago where there’s a lot of trees and a lot of food scraps in the trash, Beveroth said.

But then the West Nile virus arrived in Illinois. The virus had been circulating in Africa and the Middle East for a few decades, but the strain that appeared in the United States in 1999 was deadlier to both humans and birds.

By 2002, just a year after the West Nile virus was first detected in two Chicago crows, an estimated half of all Illinois crows had died from the virus, said Beveroth.

“Initially I thought it probably wouldn’t be a huge issue because there are lots of diseases that pop up in birds,” said Michael Ward, a senior ornithologist at the Illinois Natural History Survey. “But I was wrong on multiple counts with the West Nile.”

Throughout 2002, ornithologists like Ward were witnessing major die-offs — not just of crows, but also of jays and robins in the state. And the disease wasn’t just striking the bird population.

Why was Illinois hit particularly hard by the virus?

In 2002 alone, more people got sick and died from the West Nile virus in Illinois than anywhere else in the country.

Scientists have a few theories about why Illinois was particularly hard hit.

The first theory has to do with how viruses spread between birds. The West Nile virus mainly lives in birds, and birds of different species can spread it to one another. (It’s very unlikely that a human could catch West Nile directly from a bird. People get the virus from mosquitoes that have bitten infected birds).

And although crows are the animal species most likely to get sick and die from West Nile, they’re what’s known as a peripheral, or “dead end” host. The virus kills crows really quickly, which actually makes it harder for the virus to mutate and spread.

It’s another bird, the American Robin, that’s known as the West Nile “super spreader.”

“We know that robins don’t die from it like crows do, and so because they don’t die and every year they have more babies that are susceptible to West Nile virus, that maintains the cycle of the virus,” Ward said.

By analyzing the blood in mosquitoes’ stomachs, scientists also determined that mosquitoes prefer robin blood. Mosquitoes seem to feed on robins more than other species, even if robins aren’t the most common bird in the area, Ward said.

There are large roosts of robins that return to Illinois every year. Because they aren’t as likely to die from the virus, there are more opportunities for a mosquito to bite an infected robin and then bite a human and spread the virus.

Combine Chicago’s dense mosquito population with the high number of super spreader robins and experts say you’ve got a recipe for high rates of infection.

But scientists like Ward said that it’s also possible Illinois didn’t actually have more cases of West Nile than other states. Illinois might just have better data.

“We know [the virus] was particularly intense in the Chicagoland area, but we also do a lot of bird monitoring so we may have been more equipped to determine the impact,” he said.

Keeping track of the crows

So why does Illinois have such good data on crow populations in the first place?

The process of counting crows in Illinois is more complicated than you might think.

Illinois has been keeping records of the state’s natural resources for over 160 years, but it was in the 1970s that a scientist named Vernon Kleen began recruiting volunteers to count birds.

At the time, Kleen was struggling to compile accurate records on the state’s bird populations because different counts used different methods.

So in 1972 he recruited 650 volunteers in 62 counties for the state’s first spring bird count. Volunteers and scientists spread out across the state to note down the different birds they see and hear. And they’ve continued to do so every year since.

Scientists at the University of Illinois use that data to build estimates of how many birds of each species are in the state — and having 50 years of consistent data gives Illinois a big advantage in determining trends.

“It’s really fun to collaborate with so many fun and interesting people that care about birds,” said Beveroth, who runs the spring bird count with Ward.

“I get so excited,” she said. “I can’t sleep well ever the night before.”

For this year’s count on May 7, she’s expecting more than 1,000 volunteers to spend all day searching for birds across all 102 counties.

And it’s because of this data collected each year that scientists know the state’s crow populations have been steadily increasing over the past decade.

“They’re doing good,” said Beveroth.

So what happened with West Nile in Illinois? Why have cases decreased?

Illinois has had mosquito abatement districts since 1927, when state officials created them in response to malaria outbreaks. In these abatement districts, workers hunt down areas where mosquitoes could lay their eggs, kill the larvae and regularly spray insecticide.

In response to West Nile, the City of Chicago increased its mosquito response, and mosquito abatement districts across the state saw an increase in membership.

That’s one reason scientists say West Nile infections have decreased for both humans and birds. From there, there are two theories about what else might have contributed to the decrease in cases.

It’s possible the virus has mutated, said Beveroth. Scientists believe this mutation may lead to lower viral counts, which means there is a lower amount of the virus in a bird’s blood. This gives the bird a better chance of survival and also decreases the likelihood of a mosquito passing the virus on to humans.

There’s a second theory as to why the number of infections has decreased in the birds. Scientists know that crows have started developing antibodies, which means their immune system is learning how to fight the virus.

Scientists in Illinois continue to trap and test the state’s mosquitoes for different diseases transmitted by mosquitoes. There are now more than 50 different species of mosquitoes in Illinois, and Chris Stone, the director of the Medical Entomology Lab at the Illinois Natural History Survey, said each one presents different challenges and potential health risks.

This year, Stone’s team is paying special attention to a species known as the black-tailed mosquito, which can infect humans with Eastern equine encephalitis virus, a virus that can be more dangerous than West Nile.

“It’s very rare, but it’s been expanding its geographic range,” he said. “So we do have some concern as to whether it could at some point pop up in Illinois because we’ve seen it in neighboring states like Indiana and Michigan and Wisconsin.”

While mosquitoes have posed major health risks to humans for centuries, Stone hopes that the COVID-19 pandemic has helped Americans realize the importance of understanding and fighting viruses.

“It seems like it’s a matter of time until the next big virus comes along and it might well be a mosquito borne virus,” he said. “Due to COVID, everyone has seen how easy it is for the next big virus to come along and how dramatic those impacts could be.”

What can you do to stop the spread and protect yourself from viruses like West Nile?

Just like with COVID, there are things you can do to protect yourself from West Nile virus.

The first is to check your yard, patio and neighborhood for standing water. Many different species of mosquitoes lay their eggs in water, and it only takes a small amount of undisturbed water for the larvae to grow. Public health organizations recommend checking for clogged gutters or forgotten buckets every spring. If you keep any tires, buckets, planters, toys, pools, birdbaths, flowerpots or trash containers outdoors, rinsing them out once a week during the spring and summer can make a big difference.

In Illinois, mosquito season is mid-April to October, though they mostly bite humans in July and August. Wearing long sleeves and pants in a light color, like white, can help prevent bites. Bug spray is also a good option, although it’s important to check that the product is proven to repel insects and that it’s safe for children.

Finally, scientists recommend reporting any dead birds to the health department.

“That crow is an indicator species,” Beveroth said. “That crow told us, ‘Look, there’s this virus that’s happening, and it can potentially do damage to people,’ … it’s almost like a pre-warning.”

Your tip just might help keep Illinois — and its crows — healthy.

More about our question-asker

Phoebe Murtagh studied environmental science in college and has volunteered with the Chicago Academy of Sciences’s Chicago Conservation Corps, so the natural world is always on her mind.

“I think in general, people forget that even in a city, we are in nature. I mean, our motto is city in a garden,” the 26-year-old Pullman resident said.

Murtagh likes to cut through a park during her commute to the small nonprofit where she works. And her other job as a dog walker gives her lots of time to admire the city’s parks and natural areas.

Now that she knows what they’ve been through, she said she’ll take extra time to appreciate Chicago’s crows.

Claire Caulfield is a freelance reporter and audio producer. Follow her @CaulfieldCM