Chicagoan Katie Call likes birds. She grew up birdwatching with her dad, and her sister is a bird researcher. So while you might not know it, Katie certainly knows that twice a year, Chicago sits under a moving cloud of birds.

Spring and fall migration bring a huge number of birds over, around, and through Chicago. Tens of millions of birds. Flocks that would make Hitchcock tremble.

And Katie knows some of those birds get confused by the glass and lights of the Chicago skyline, smash into skyscraper windows and then die. So twice a year — several years going — she’s wondered:

How many migratory birds die by crashing into Chicago skyscrapers each year?

It’s not just her concern for the cute little birds. Katie wants to know how birds dying in Chicago might affect their populations.

So her question is a two-parter: First, how many migratory birds die hitting those skyscraper windows on their way through Chicago? Second: Is that number high enough that we should worry about the bird population?

Well, we do have a number in hand. It’s only an estimate, really, since there’s no decennial bird-death census. But (spoiler alert!) the number’s an interesting one; it’s modest enough to give bird well-wishers a sense of relief, but not so small to leave bird well-wishers feeling entirely comfortable, either.

How many dead birds are we talking about?

Doug Stotz, a conservation ecologist at the Field Museum of Natural History, has spent decades studying bird migration through Chicago. And he doesn’t have to walk very far to find specimens; he and his team check daily for dead birds outside of McCormick Place.



“Over the years, around 37,000 birds have hit that one building,” Stotz says. “I suspect the tall skyscrapers downtown are probably killing something on the same order, about 1,000 birds a year. So it’s tens of thousands in downtown.”

Annette Prince, the head of Chicago Bird Collision Monitors, backs Stotz’s estimate. Her group, which seeks to identify the causes of bird strikes as well as prevent them, has volunteers patrol downtown Chicago for dead or injured birds. (They send injured birds to Glen Ellyn’s Willowbrook Wildlife Center, while the dead birds head to Stotz at the Field Museum.)



Prince says her group finds 5,000 dead birds in downtown Chicago each year. Regardless, Prince says, she knows that number is the tip of the iceberg.

“That’s just one mile of the lakefront,” she says. “You multiply that out, you get into the tens of thousands of birds. It has to be that number.”

Chicago: Bird oasis in a sea of corn

So we’ve got an estimate: Each year tens of thousands of birds die hitting skyscraper windows downtown. If that figure sounds high to you, let’s put it in perspective. There couldn’t be that many bird deaths without a good number of live birds, right? And the city, it turns out, is relatively hospitable to migratory birds.

Chicago sits on what’s known as the Mississippi Flyway, a veritable bird superhighway.

“We think of Chicago as being as being all gray, with buildings and streets and so on,” Stotz says. “There’s a huge number of trees here compared to the agricultural counties, which are mostly corn.”

All told, about 40 percent of all North American migratory birds use the Mississippi Flyway each year. That bi-annual exodus involves more than 200 species.

For some species, the migration is a short hop. The Yellow-Bellied Sapsucker, for example, flies from the southern U.S. to just north of the Great Lakes. On the other extreme, the Connecticut Warbler flies from the jungles of South America to the Canadian tundra — and back! — every year.



At some point, many of the billions (yes, with a “B”) of birds migrating through our region have to navigate Chicago’s architecture. And for a fraction of those flying through, that means untimely death.

Comparative mortality

But, remember: Katie’s not just in interested in the numbers — she wonders whether we should worry about the Chicago’s skyline’s effect on entire bird populations.

Stotz says it’s hardly an existential threat to the birds, adding that the city skyline is just one hurdle among many, including starvation, storms, habitat loss, and predators.

“Migration is an inherently dangerous thing that birds are doing,” he says.

He remembers one night a few years ago, when a huge number of birds were passing over Chicago during a storm.

“They came together out over the lake and so the birds got forced down into the lake, where they drowned,” Stotz says. “We checked the beaches, and my memory is we found about 4,000 recently dead birds.”

That’s just the kind of thing that happens when billions of birds migrate. Even these huge, naturally-occurring die-offs don’t seem to make a difference in populations.“Bird biology is sort of structured to accept the fact there will be a lot of mortality associated with migration,” Stotz says. “For a common, widespread bird that’s not under other pressures, it’s probably not a conservation issue for them. It’s still sad, but it’s not a conservation issue.”

Still, Prince worries bird collisions could — at some point — create a tipping point for some species. Climate change has thrown a wrench into the migratory patterns of the ovenbird, for example, and every winter, the Scarlet Tanager finds less and less of its habitat in the South American jungle.



“Some of them can ill-afford losing even more members of their species,” she says. “It’s not something you’re going to notice right away, but at some point, if the entire species loses its strength to survive and we’ve done this by adding these urban hazards, it could have severe consequences for many of the species that we’re dealing with.”



A bright side to local bird deaths?

Stotz doesn’t like that so many birds die hitting windows, but he says it’s worth studying the phenomenon.

Most bird research is done at the healthy remove of a set of binoculars, but that can only tell you so much. Because the scientists the the Field Museum have decades-worth of dead specimens to measure, they’ve noticed several trends. For one, birds’ bodies are shrinking, while their heads have largely stayed the same — a shift that researchers believe may be an adaptation to a warming climate. And patterns gleaned from collections of dead birds have proven that migratory patterns are shifting in response to climate changes.

Stotz’s hope is that the long-term data can help identify the biggest bird killers, like habitat loss, forest fragmentation, pollution, and climate change.

“It makes me sad to think of all these birds, especially in the spring. They made it through fall migration, they made it through the winter, and they’re almost ready to get to the breeding grounds,” Stotz says. “And they run into McCormick Place or the Hancock or the window in my backyard. But we try to take advantage of that.”

What you can do



If you’re with Katie, our questioner, and you feel bad about the number of birds that die because of humans, here are a few things to consider.

Any window can kill a bird, not just the ones in skyscrapers. Taping patterns onto your own windows during migratory seasons, or even just leaving them dirty, can make the windows less confusing for birds.

Turning off or dimming your lights during migratory season can have a big effect. The Chicago Collision Monitors and other advocates pushed for the Lights Out Chicago program, which encourages downtown buildings to turn off or dim their lights during migratory season. That move alone dropped the number of bird strikes by 80 percent.

But if you’re looking for the best bang-for-your-buck way to cut down on bird deaths, you’ve got to look at the biggest human-related killer of birds.

Not windows, but cats.

Keep your cat in check. Cats kill about 2.5 billion birds every year across the U.S. So, keeping your cat inside for a few weeks in the spring and fall might be the most cost-effective way to cut down on bird deaths.

More about our questioner



Katie Call lives in Chicago’s Lakeview neighborhood. She gets her love of birds from her dad, who taught her how to band migrating woodcocks in her home state of Michigan. Apparently he set quite an example; Katie’s sister happens to be the raptor specialist for the state of Maine!

While she’s admittedly not a morning person, Katie joined Curious City for a gruesome bird hunt, to learn how bird collision monitors do their work. We didn’t spot any dead birds ourselves, which is probably for the best.

She says while Chicago’s buildings might not be directly driving birds to extinction, she wants our city to kill fewer birds.

“I definitely want to be involved in some way now that I know about this. Other than the early morning route. Which… maybe,” she says, laughing. “Maybe I’ll do that.”

Sean Kennedy is a freelance reporter in Chicago. He tweets at @stkennedy. Send him bird questions.