“Naturally Philly” is a Billy Penn column that explores the city’s wildlife and plants — both familiar and extraordinary — that thrive or struggle in our urban environment.

From the sidewalk, it’s inspiring to watch the rise of new buildings on the Philadelphia skyline. But from above — say, a bird’s-eye view — the urban towers of glass and light have a different impact.

Collisions with buildings, both high and low, are a leading cause of bird deaths in the city.

Of course, this isn’t just a Philly thing. Across the country, window collisions kill nearly a billion birds each year, the U.S. Fish and Wildlife Service estimates. About half involve skyscrapers.

Keith Russell, a Philadelphia-based ornithologist with Audubon Pennsylvania, has studied the problem in Center City and beyond. It’s not that birds can’t see glass, Russell explained. They just don’t know what it is.

“The problem is, most birds are raised in areas where there is no opportunity to become familiar with it,” he said. “So when they start migrating and get into a city, their first encounter with glass is often their last.”

From 2008 to 2011, Russell worked with the Philadelphia Zoo and his former employer, the Academy of Natural Sciences, on a study that monitored bird strikes in the heart of Center City. It focused on a 3.5-square-block area from 17th to 19th Street and Market to Arch.

“We estimated that a thousand birds a year were colliding with buildings in that one study area,” he said. “That’s a lot.”

Dozens of species, millions of birds

Around 75 different species were represented among the dead or injured birds — three-fourths were dead — found during that study, a diversity that surprised even Russell.

Over the years, he and others have found a number of unusual species among birds that have collided with Philly buildings.

There was the body of a wild turkey discovered on North Broad. A saw-whet owl, the smallest owl in the East, on 19th. A yellow-throated warbler, a rare visitor to the city, on a downtown sidewalk. A yellow rail that had flown down a stairwell and into a glass door at a large building in North Philadelphia.

“I’ve still never even seen a yellow rail in the wild,” Russell said. “They’re so secretive, and there aren’t many of them.”

By measuring the distance from the nearest building to where the birds were found on the sidewalk or street, and by visiting the sub-roofs of high-rises, the research team determined the collisions were occurring at all building levels.

The study found that most of the bird collisions happened during migration. From April to May and September to October, tens of millions of migrating birds take the Atlantic Flyway through Philly each year.

“They use every opportunity to rest and feed and every tree. It’s really important to have places for them to stop,” Russell said. “Unfortunately, when you have lots of vegetation in an area that is adjacent to these buildings, it becomes a killing zone.”

‘Massive kills’ around City Hall

The issue isn’t unique to the modern skyscraper era. Along with glass, lighting can play a big role in causing bird collisions.

A hundred years ago, when City Hall dominated the skyline, bright lights illuminated the Billy Penn statue and the central tower. On foggy or rainy nights, when migrating flocks got disoriented, there could be “massive kills” on City Hall, Russell said. Workers would discover the bodies in the morning, and incidents were reported in the journal of the Delaware Valley Ornithological Club, which was formed in 1890 and still follows bird occurrences in the region.

In the 1930s, the Washington Monument, a lone beacon on the National Mall, was another site of frequent bird collisions. “There are no windows involved there. The birds were disoriented by the bright lights” shining on the monument, Russell said.

Lights confuse birds by reflecting images on glass surfaces or by disabling the sensory pathways birds use to navigate.

Birds navigate by the position of the stars and landscape features, and by sensing the earth’s magnetic field. Certain colors of lights can disable their ability to detect the field, and lights in general may exacerbate birds’ perception of reflective glass.

A new kind of glass

Expanding campuses, with new glass towers rising each year, are obvious danger zones for birds. But schools also have the financial resources and a strong interest in adopting methods of addressing the problem.

In conjunction with the big Center City bird collision study, researchers also looked at Temple University’s Broad Street campus.

Working with student volunteers and Temple grounds crews in spring 2009, the monitoring team counted 53 bird strikes in one month at buildings with highly reflective glass and dense vegetation around them. A longer study of collisions found dead birds representing 40 species around Temple.

Temple has tried several interventions, said Kathleen Grady, the school’s director of sustainability.

First they mounted models of hawks atop several buildings to frighten birds away. It didn’t work. Next, they applied a film with a pattern that birds could detect on windows of high-impact buildings. That did work, so a student graphic design competition was held to come up with alternate bird-friendly patterns. The winner was installed on several buildings.

Another successful approach was the installation of netting over the front of buildings, which acted kind of like like a bird trampoline. It had the benefit of being removable when migration periods ended, but was challenging to use on taller buildings.

The solution for new buildings has been fritted glass, which has patterns visible to birds as well as people. Temple used fritted glass on the 27-floor Morgan Hall residence — to great effect.

“We still have occasional collisions on the building, but it’s five per season instead of 35 on other buildings,” Grady said.

Using fritted glass is considerably more expensive, Grady noted, but it also reduces solar heat and cooling costs.

Meanwhile, Russell and his Audubon colleagues are working closely with the University of Pennsylvania to reduce bird collisions on its West Philadelphia campus, where the problem was especially acute on the glass bridges connecting buildings, explained Bob Lundgren, Penn’s landscape architect.

A pilot study using patterned film on the glass “essentially stopped the collisions there,” Lundgren said.

The university now invites citizen-science observations via its Penn Bird Strikes Project, and is getting proposals for the application of film to older buildings. Since the alteration of architecturally significant structures can be “touchy,” Lundgren said, the university is working with architects to utilize patterns that are suitable to the original designs.

In newer Penn buildings, fritted glass is being used — both for the energy savings and the bird-friendly aspect.

Both universities are now including conversations about bird strikes in the building design process.

DIY solutions

In addition to adding patterned film to windows, building and home owners can hang cords or other materials in windows to make them more visible to passing birds.

The spacing between cords or other vertical elements in a visual barrier should be four inches, according to Dr. Dan Klem, of Muhlenberg College, who has been conducting the groundbreaking research on bird strikes since the mid-1970s, Russell said. Klem and Muhlenberg’s Acopian Center for Ornithology offer a variety of techniques to reduce bird collisions.

There are streetwise birds that have developed an awareness of glass. At his home in Germantown, Russell watches mourning doves fly down to sit on the air-conditioner outside his office window. “They know I’m there, and they know there is glass there. They know what it is.”

The city’s omnipresent house sparrows, attracted to the wild bird seed inside, have learned to fly in and out of the automatic glass doors at the Home Depot. “They get it,” Russell said.

“But most birds are never going to learn what glass is. So it’s up to us to make the effort to protect them.”