At 5:30 a.m., the rain was letting up, but it was still dark as the two men began their rounds of Center City's skyscrapers.
They started with a particular alcove, bordered on three sides by glass.
Pretty. But not for birds.
"They get trapped into this angle," said Stephen Maciejewski, scanning the sidewalk for victims. Confused by all the reflections, "they don't know to turn around."
So they fly into the glass, and they die.
As they walked from building to building, he and Keith Russell, Audubon Pennsylvania's science and outreach coordinator in Philadelphia, checked spots where they usually find birds - along sidewalks, behind signs, in stairwells, under cars, atop ledges.
During spring and fall migrations, from 2008 through 2011, Maciejewski, Russell, and others regularly walked a study area from Market Street to Arch and 17th to 19th, collecting birds and data.
By quitting time at 8, they might have picked up a dozen birds or more. Extrapolating from that and other data, Russell concluded that 1,000 birds were dying after colliding with buildings in that area each year.
Muhlenberg biology professor Daniel Klem Jr., regarded as the national expert on bird strikes, has estimated that a billion birds die every year in the United States as a result of striking buildings.
A hall of mirrors
Look up in Center City, and it's mostly reflections.
You could liken it to a hall of mirrors, but there's also a lot of transparent glass, and that's just as bad, Russell said. Birds see a tree on the other side of an atrium and try to get to it.
"We've seen birds fluttering against the glass, trying to get in," he said.
Researchers know it's not just Center City buildings, and not just these buildings. Just about any size glass window can kill a bird.
Cities such as Chicago and Toronto - perhaps the first place that many migrating songbirds encounter as they leave the boreal forest where they breed - have been studying the issue and tallying the kills since the 1990s.
Recently, suits were brought against two owners of office buildings in Toronto. One was accused of violating Canada's Species at Risk Act. The other was accused of violating Canada's environmental laws by "discharging a contaminant" - reflected light.
Researchers used to think that birds hit mostly at night, and mostly on lower floors. But now they're finding that birds hit 24 hours a day and at all levels.
It's not the resident birds - the house sparrows that chitter from the trees and pigeons that patrol the sidewalk in search of crumbs. They've learned the terrain.
It's the migrating birds that die. The most common are oven birds, common yellowthroats, and white-throated sparrows.
But the searchers find inexplicable victims, too, such as woodcocks, birds that breed in forests. Two wild turkeys have hit windows in Philadelphia, one of them breaking the glass.
It's mostly young birds that hit, possibly because their navigation systems are still developing. Russell's crew also has found clusters of dead birds that were all the same species, telling him that birds are making choices.
"They're not just flying by and hitting a building because they didn't see it," he said. They're attracted by something that they interpret as a possible refuge or food source; then they get in trouble.
For years, the conventional wisdom was that building lights were the problem.
Locally, the theory extends back to the 1890s, when City Hall got outdoor lights. "Almost immediately, people started finding dead birds," Russell said. Especially in fog or rain. Birds would circle the lights and hit the building.
Based on that knowledge, Russell and the Philadelphia Zoo's conservation director, Valerie Peckham, went to the city's Building Office Managers Association to ask its members to turn out some lights.
Bad timing. The city had just celebrated "Light Up, Philadelphia," a plan for creative lighting, Peckham said. Plus, the association wanted data. Where was the proof?
So with funding from the U.S. Fish and Wildlife Service and Toyota's "Together Green" program, Audubon began the survey.
Official survey work has ended, but Russell and Maciejewski still check from time to time.
And Russell still thinks that indoor lights left on at night are an attraction for birds, which may see something inside they want to get to.
What can be done?
Researchers are trying to find solutions.
Films applied to the glass to make it visible are one idea. At Temple University, art students held a competition to come up with attractive and effective designs. The winning version resembled sheet music with the notes shaped like birds.
Another student cut translucent film into the shape of molecules for the chemistry building's windows.
A New York company, SurfaceCare, provided a film with small, almost imperceptible, black stripes for the Bear Country exhibit at the Philadelphia Zoo, which, like many other zoos, has a combination of glass enclosures and trees. The bird strikes stopped.
SurfaceCare owner Marc Sklar is looking at new technologies, including glass that has elements in the ultraviolet light spectrum, which humans can't see but birds can.
Christine Sheppard, a bird collision expert with the American Bird Conservancy, has been testing prototypes of bird-friendly glass at a bird-banding station near Pittsburgh. She said the results could be used to write guidelines for architects.
"Nobody wants to kill birds," she said. But, still, "nobody wants more rules and regulations." Her goal is to educate.
"There are lots of beautiful buildings that are very bird-friendly," she said. "You can be creative and do architecture that will win awards" and still not kill birds.
Recently, the national green building certification program - Leadership in Energy and Environmental Design (LEED) - began a pilot program that gives buildings points for having bird-friendly windows.
"So-called green buildings . . . are never green to me, no matter what their LEED award, if they kill birds," Muhlenberg's Klem said.
Chicago and San Francisco require that new buildings and major renovations incorporate bird-safe elements.
Minnesota requires that all state buildings turn off lights during migration. Michigan's governor issues an annual proclamation declaring migration "safe passage" dates and asking that buildings remain unlit at night.
A year ago, Illinois Congressman Mike Quigley introduced national legislation, still pending, to mandate bird-friendly construction for federal buildings.
All of the Philadelphia specimens that the researchers have collected are in the ornithology collection of the Academy of Natural Sciences.
Collection manager Nate Rice opens drawer after drawer in a special cabinet. "Warblers, sparrows, thrushes," he says. "Flickers and blackbirds and woodpeckers."
Many of the academy's other specimens have tags noting exotic locales: the Everglades, the Amazon, some remote Pacific isle.
But the tags on these new birds indicate more familiar realms: 1800 JFK, 20th and Race, Peco building, Comcast building.
To Rice, these specimens are a repository of potentially valuable data about genetics, diet, contaminants, and the environment they lived in.
Back out in Center City on that rainy morning, Russell and Maciejewski stopped their search at 7:30. They figured the rain had kept the migrants grounded, and they'd found no birds. But they knew not to put too much hope into that.
On a previous day, Maciejewski had walked for nearly three hours and, to his relief, had not found any dead birds.
Before heading home, he paused in front of a window.
Just as a bird hit the glass.
"It made a loud thud and dropped in front of me," he said. "In seconds, it was dead."
This article has been revised to reflect the following correction. The number of birds killed each year by striking buildings in the United States is one billion, according to Muhlenberg biology professor Daniel Klem Jr.