JOHN YANG: Every year in the United States, as many as a billion birds die while migrating.

It's not due to climate change or some toxic substance in the air or water.

This culprit is hiding in plain sight.

The birds are killed when they collide with glass.

For the latest in our Saving Species series, we went to see an effort to reduce this carnage in Philadelphia where it turns out, residents love a lot more birds than just the Eagles.

STEPHEN MACIEJEWSKI, Volunteer, Bird Safety Philly: And so the birds are attracted to the brightness.

JOHN YANG (voice-over): The sun is barely up over downtown Philadelphia and the guy known as the Birdman is already at work.

Stephen Maciejewski is a fixture here during the spring and fall migrations, looking for birds not in the skies, but on the sidewalks where they fallen after hitting a window.

Some are dead, some are injured, others just stunned.

STEPHEN MACIEJEWSKI: They don't know what glass is.

They don't know glass is hard.

They just say reflection or they think they can go in or through.

And then they start hitting.

JOHN YANG (voice-over): It didn't take long for Maciejewski's eagle's eyes to spot a dead ovenbird, a small songbird.

STEPHEN MACIEJEWSKI: So beautiful.

Yes, there's orange crown, and they have this loud voice singing teacher, teacher, teacher.

JOHN YANG (voice-over): The species and other details go into his logbook.

The dead bird goes into a plastic bag to be taken to the lab for study.

All too common routine done, he moves on.

Maciejewski is a volunteer for Bird Safe Philly, a partnership aimed at creating safe spaces for birds.

Since 1970, the United States and Canada have seen a decline in bird species of 30 percent.

While much of it's due to habitat loss, glass windows posed the third largest threat.

Robin Irizarry is with Audubon Mid-Atlantic.

ROBIN IRIZARRY, Audubon Mid-Atlantic: Folks don't recognize just how serious of a threat this is because it's such a passive threat.

It's not something that we're actively doing like engaging in deforestation or, you know, paving over areas of habitat.

This is simply by us existing in the way that we live having windows.

It's a threat to birds, and it's killing birds.

JASON WECKSTEIN, Academy Of Natural Sciences at Drexel University: Sort of organized how we classify birds.

JOHN YANG (voice-over): So Jason Wettstein, runs a lab studying birds with the Academy of Natural Sciences of Drexel University.

JASON WECKSTEIN: And you can see this is filled with specimens with Safe Philadelphia.

JOHN YANG (voice-over): It's collection of the birds killed in windows strikes dates back nearly 150 years.

JASON WECKSTEIN: If you look here, you know, they're specimens from 1877.

JOHN YANG: Wow.

JASON WECKSTEIN: This one from 1906 is actually from the early window kills that happened here in Philadelphia.

So this was on city hall.

When City Hall was first lit up, we actually had windows strikes happening or, you know, there were some window strikes just from glass alone.

That does happen.

And certainly glass on its own is a problem when it's reflective.

But you add lights into the mix, and it makes it a lot worse.

JOHN YANG (voice-over): Birds make their long migratory journeys mostly at night.

And for reasons still not fully understood, they're drawn to artificial lights.

Philadelphia sits on a migratory superhighway known as the Atlantic Flyway, so it's brightly lit skyline is ripe for the sort of mass collision that occurred on October 2nd, 2020.

STEPHEN MACIEJEWSKI: We had around 450 birds just at this building.

And, you know, I take care of like a number of birds.

I process them.

And people say there's more down there and more down there.

JOHN YANG: How many birds did you collect that day?

STEPHEN MACIEJEWSKI: Eight or 900.

JOHN YANG: Wow.

STEPHEN MACIEJEWSKI: And we kind of thought that there was probably over 1,500 that were killed.

MARIANNE SCOTT, Building Industry Association of Philadelphia: And that happened essentially my backyard.

I live in that area.

JOHN YANG (voice-over): That catastrophic event move bird lover Marianne Scott to action.

MARIANNE SCOTT: There is a general misconception that everyone -- it's the high rises that are the biggest problems.

So, yes, the statistics I think is that it's four stories or less where the most fatal collisions occur.

JOHN YANG (voice-over): As Executive Director of the Building Industry Association of Philadelphia, she's working to convince the members of that residential real estate trade group to turn off lights both inside and outside between midnight and 6:00 a.m. during migration seasons.

MARIANNE SCOTT: I think by and large people don't know that there's a problem.

They're shocked to hear when I say there might be 300 million birds traveling across the country now.

JOHN YANG (voice-over): In fact the night before we went on patrol with Stephen Maciejewski, an estimated 660 million birds were in the skies over the United States in the midst of their spring migration north.

That makes for a busy morning.

An injured ovenbird is placed in a paper bag for transport to the School of Wildlife Centers Clinic for rehabilitation.

Then Maciejewski helps and apparently stunned birds \struggling to regain its sense of direction and get on its way.

Next, an injured yellow throated warbler.

And at the entrance of an office building, a box safeguarding the body of a dead bird left by building staff who've gotten to know much Maciejewski.

OLIVIA FERMANO, Building staff: I actually have him in my contacts on my phone because this is sadly regular occurrence.

There's been times when I've been sitting there and they've flown in, you know, to the window.

DON HAAS, BOMA Philadelphia: Really all the building staff whether it's security or janitorial predominantly, who used to just clean up and move on.

Now we're trying to save as many as possible.

JOHN YANG (voice-over): Building Manager Don Haas is a key leader in the Building Owners and Managers Association of Philadelphia or BOMA.

He's working to get his counterparts on board.

DON HAAS: We were controlling all the outside lighting in the crown lighting, lobby lightings.

So we encourage all of our BOMA members to sign on to the Birds Safe Lights Out Philly Program.

We had a terrific response.

I think we're probably at 60 buildings.

JOHN YANG (voice-over): Some cities and states require bird safe design.

In Maryland state law says buildings that are at least 50 percent publicly funded must follow bird safe design practices.

On New York City law mandates that all city owned buildings turn off lights at night during migration season.

In Canada, Toronto requires new and existing buildings to take steps to be bird friendly.

In West Philadelphia, the University of Pennsylvania makes being feather friendly part of its sustainability plan.

Many school buildings take part in Lights Out Philly.

And in some like the Singh Center for Nanotechnology, window glasses etched with vertical stripes, a pattern that's been found to reduce collisions.

It's the same ideas the dots on the transparent film covering the windows of Water College House one of the campus dorms.

ZADE DOHMAN, Student, University of Pennsylvania: I've sort of become the bird expert in my friend group.

I received tax where friends will say, I heard this bird at this late hour of night is this normal.

JOHN YANG (voice-over): Student eco rep, Zade Dohman, monitors the campus for bird strikes as part of his work study program.

ZADE DOHMAN: There have been birds I've picked up right after they were hit.

And most of them were dead.

So I can feel the sort of remnants of life leave their body.

And it is really hard.

And it's not something that gets any easier.

To me, it's always struck me as this sort of microcosm of how humans interact with the environment.

In general, it's our architecture, it's our human influence on the environment around us that is causing these birds to get hurt and to die.

JOHN YANG (voice-over): It's an idea that helps inform the work of university landscape planner Chloe Cerwinka.

CHLOE CERWINKA, Landscape Planner, University of Pennsylvania: The way I got interested in birds was through trying to find another way to understand whether our landscapes were functioning because if we're inviting these birds in by creating these important habitats that they need, then we absolutely have to make sure that it's a safe space for them.

We can't invite them in just to let them die in the built environment.

JOHN YANG (voice-over): The solutions Penn is employing are just some of the methods found to be effective.

At the Discovery Center in Philadelphia, Robin Irizarry shows us others.

ROBIN IRIZARRY: These are Acopian birds saving blinds.

These are a simple technique, a really cost effective technique.

And this is just a network of paracord that protects birds from flying into the glass here.

You want to have a distance of about four inches.

So when a bird is flying through there, it's not going to feel comfortable going through that space.

STEPHEN MACIEJEWSKI: I like they call me Birdman.

It's an honor.

JOHN YANG (voice-over): Back on the streets of Philadelphia, Stephen Maciejewski wraps up his day.

He's collected 13 dead and injured birds.

The injured were sent to rehabilitation.

The dead were neatly placed in a freezer at the Academy of Natural Sciences alongside other birds that have met similar fates, all available to be used for research.

STEPHEN MACIEJEWSKI: It's very sad to find these birds.

I mean, they're coming up north looking to raise a family, traveling like thousands of miles from, you know, South America, Central America, the Caribbean to come this far and then to die on the streets of Philadelphia.