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,” said Murtagh, 26.
The lifelong Chicagoan spends a lot of time in parks and loves watching the large, black birds explore the treetops. But she doesn’t remember seeing many crows growing up.
“In the past five, 10 years, I see [crows] a lot more than I can ever recall,” said Murtagh, who studied environmental science in college and has volunteered with the Chicago Academy of Sciences’ Chicago Conservation Corps.
She wondered whether the number of crows in the city has actually increased.
It turns out they have. There are more crows in Illinois today than 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
This story originally appeared on WBEZ’s Curious City, a podcast that answers people’s questions about Chicago and the region.
It goes back to 2001, when scientists discovered two dead crows in Chicago that tested positive for the West Nile virus.
Though most people only suffer mild illness, West Nile can be deadly. Since it first was detected in the United States, more than seven million people have been affected by the virus.
It also decimated the Illinois crow population, which 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 downstate Danville 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 with 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 are a lot of trees and a lot of food scraps in the trash, Beveroth said.
Then, West Nile arrived in Illinois. The virus had been circulating in Africa and the Middle East for 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, it was estimated that half of all Illinois crows had died from the virus, according to 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 with the Illinois Natural History Survey. “But I was wrong on multiple counts with the West Nile.”
Throughout 2002, ornithologists in Illinois were witnessing major die-offs — not just of crows but also of jays and robins. And the disease wasn’t just striking the bird population.
Why West Nile hit Illinois hard
In 2002 alone, more people got sick and died from West Nile in Illinois than anywhere else in the country.
Scientists have a few theories about why Illinois was particularly hard hit.
The first 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 West Nile 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,” Ward said. “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.”
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 it’s also possible Illinois didn’t actually have more cases of West Nile than other states but instead 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?
The process of counting crows 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 only 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 the different birds they see and hear. They’ve continued to do so every year since.
University of Illinois scientists use that data to estimate 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, I can’t sleep well ever the night before.”
For this year’s count this weekend, she was expecting more than 1,000 volunteers to spend all day searching for birds across all 102 counties.
It’s because of the data collected each year that scientists know the crow populations have steadily increased over the past decade.
“They’re doing good,” Beveroth said.
Why has the number of cases fallen?
Illinois has had mosquito-abatement districts since 1927, when officials created them in response to malaria outbreaks. 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 the number of West Nile infections has decreased — for humans and birds.
There are two theories about what else might have contributed to the decrease.
It’s possible that the virus has mutated, Beveroth said. Scientists think this mutation might lead to lower viral counts, which means there is less of the virus in a bird’s blood. This gives the bird a better chance of survival — and lessens the chances of a mosquito passing the virus on to humans.
There’s a second theory why the number of infections has decreased in birds. Scientists know crows have started developing antibodies, which means their immune system is learning to fight the virus.
Scientists continue to trap and test mosquitoes for diseases they transmit. There are more than 50 species of mosquitoes in Illinois, and Chris Stone, director of the Illinois Natural History Survey’s medical entomology lab. Stone said each one presents different challenges and potential health risks.
This year, Stone’s team is paying special attention to the black-tailed mosquito. It can infect humans with Eastern equine encephalitis virus, which 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.”
Stone hopes 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.”
Steps to slow spread, protect yourself
As with COVID, there are things you can do to protect yourself from West Nile virus.
The first is to check your yard for standing water. Many mosquito species lay their eggs in water, and it takes only a small amount of undisturbed water for the larvae to grow. If you keep any tires, buckets, planters, toys, pools, birdbaths, flowerpots or trash containers outdoors, rinsing them out once a week during spring and summer can make a big difference.
In Illinois, mosquito season is mid-April to October, though they bite humans mostly in July and August. Wearing long sleeves and light-colored pants can help prevent bites. Bug spray is also a good option, but it’s important to check that the product is proven to repel insects and is safe for children.
And scientists recommend reporting any dead birds to your 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.”
