As countless birds are now finishing their spring migrations, it can be a dangerous journey, in part because of man-made obstacles.
In the U.S. alone, hundreds of millions of birds die each year after flying into buildings. In fact, the phenomenon has drastically reduced the population of some bird species.
A group in Philadelphia is doing something about it.
Before sunrise, Audubon Society volunteer Stephen Maciejewski searches Center City, Philadelphia for fallen birds.
"During the night, it's very dark," Maciejewski told Brook Silva-Braga. "So they see that light, they wanna get in there for whatever reason. They don't understand glass is hard."
Stephen's daily report gets put into a bag and then goes to Urban Conservation Program Manager of the Audubon Society Keith Russell, who has spent 15 years documenting the dead birds to push Philadelphia to help solve a global problem.
"It's a factor in the decline of lots of species," Russell said. "Since 1970, we've lost about a third of our birds."
The design of modern skyscrapers is partly to blame. They're often towers of reflective glass that look like more open sky to birds.
The problem got little notice in Philadelphia until Maciejewski made his rounds one morning last October. At one building, he said he found about 300 dead birds.
A cloudy night at the peak of fall migration caused a massacre.
"One guy brought a dust pan of birds. He dumped it out in front of me. There were 75 birds; some living, a lot of dead birds," Maciejewski said. "It was a rough day."
His photo of the slaughter made headlines that got the notice of people like Cory Gunselman, who manages the skyscraper ONE Liberty Place in Philadelphia.
"It's been a problem honestly," Gunselman said. "We do notice the collisions that do happen."
Thanks to a trade group, building managers like Gunselman finally heard Russell's pitch: They could save birds—and some energy—by turning off the exterior lighting that seems to confuse birds at night.
Gunselman added her building to the program. Lights Out Philadelphia follows similar programs in nearly 40 other American cities.
"They use the stars in part to navigate, which might explain why they're attracted to lights in some cases," ornithologist at the Academy of Natural Sciences in Philadelphia Jason Weckstein said. He added that there is reason to believe the dark sky will help the problem.
But skyscrapers make up only a tiny percentage of all buildings. Ordinary homes end up killing many more birds.
Education manager of Philadelphia's Discovery Center, Damien Ruffner, has suggestions. He says that small vinyl dots or any vertical lines that are four inches or less placed on windows can break up the reflection.
But intentionally blocking a view can be a tough sell.
"That is something that we definitely could look into," Gunselman said. "The one thing is that we don't want to take away the luster of our beautiful double-pane windows that the building is known for."
Nonetheless, Russell cares that his city finally seems to notice its birds.
"It's really amazing after having worked in this area for so long," he said. "Really warms my heart. It makes me very proud to be a Philadelphian."
There were no major bird strikes in Philadelphia this spring migration and building lights in the city will be on again soon.
But when the birds head back south in the fall, Russell already has a promise that the brightest light over center city will again be the moon.