MADISON, Wis. — Shortly after dawn on a September Saturday, Crystal Sutheimer stalked through the landscaping around Signe Skott Cooper Hall on the UW-Madison campus.

In her right hand, a net. Her eyes were on the ground. Yellow tags pinned to her backpack identified her purpose: Bird Research.

"In case I get yelled at," Sutheimer said.

Each fall and spring, Sutheimer spends six weekends combing the UW-Madison campus for dead birds, victims of collisions with windows.

Sutheimer is part of the Bird Collision Corps, a group of nearly 70 citizen scientists monitoring buildings throughout the Madison area to document the toll the urban environment takes on avian wildlife.

To a bird in flight, windows can look like open sky.

Window collisions are the second-leading cause of human-caused bird mortality, killing nearly 600 million birds a year, according to a 2015 study. Data gathered by the BCC suggest tens of thousands of birds are killed each year in Madison, the Wisconsin State Journal reported.

And while single-family homes account for almost half of window strikes, a handful of large buildings with lots of windows can kill a disproportionate number, said Brenna Marsicek, director of communications for the Madison Audubon Society.

DNR wildlife veterinarian Lindsey Long said there were no more dead birds than normal, and none of the birds submitted for testing showed lesions consistent with those of birds sampled in other states.

The idea for the collision corps was hatched in 2017 when UW-Madison hosted a neighborhood meeting about plans for the Nicholas Recreation Center. Marsicek said someone asked how the university planned to prevent birds from flying into the $96.5 million building's windows and the university didn't have an answer.

Marsicek said the Audubon Society decided a survey was needed to understand the scope of the problem. Now in its fourth year, the BCC's work spurred a bird-safe glass ordinance that is the subject of a lawsuit by real estate, development and construction groups.

The ordinance, adopted by the City Council last year, requires that buildings over 10,000 square feet and other large structures include safety features -- such as dots, lines or other patterns -- in some windows to reduce the risk of birds colliding with the glass.

The conservative Wisconsin Institute for Law and Liberty contends it runs afoul of a 2014 state law that prevents cities from adopting requirements that go beyond the minimum standards adopted by the Department of Safety and Professional Services.

The city says buildings can be brought into compliance with something as simple as stickers affixed to glass, just as UW-Madison did with its Ogg Residence Hall for about $20,000.

Developers say that might work for a walkway connecting two wings of a dorm but not in a typical building, where special bird-safe glass that's transparent to the human eye can cost up to four times as much as standard glass.

"Anything that significantly impedes the view is not going to be welcome by many tenants," said Bill Connors, executive director of Smart Growth Greater Madison.

A partnership between Madison Audubon, UW-Madison, Urban Land Interests, American Family Insurance and the Holy Wisdom Monastery, the BCC surveys 31 buildings throughout the city that have been identified as high-risk for bird mortality.

Volunteers sign up for daily slots during the fall and spring migration seasons, when millions of songbirds pass through Madison each night on their way south.

"Birds can and do hit windows throughout the year," Marsicek said. "During migration periods so many are on the move, so many who live in northern Wisconsin and Canada are making their way to their winter grounds. Lots of migrants this time of year were born this year -- birds that haven't migrated before and aren't familiar with urban settings and how to navigate around them."

Each volunteer is assigned a route with three to four buildings, which they check in the early morning hours, collecting any birds that might have fallen in the previous day, documenting the date, time, location, species and condition.

The information is then uploaded to and to the Audubon Society's own database.

Dead birds are packaged in food storage bags and left in a small freezer tucked into a crevice behind the Russell Laboratories building, where ornithologists verify the species and preserve any in good enough condition to study.

Volunteers occasionally find injured birds, which they bring to the Dane County Humane Society's wildlife center, where they are rehabilitated or euthanized.

Maggie Honig and her husband, Bob, signed up for a survey route after moving to Madison earlier this year. With backgrounds in biology and environmental sciences, both had been involved with the Audubon Society in Texas but felt the window strike surveys would have a more direct impact on policy decisions.

"Gathering data for Christmas bird counts has its benefits, but this was a little more powerful," Honig said. "It's absolutely rewarding knowing that we're out there and contributing to this."

Honig described the sadness she felt last week when they found their first dead bird, a Tennessee warbler outside the Kohl Center.

"For them to take off on this incredible journey and have it all come to an end -- it's very hard to look at a dead bird on the ground and not feel the enormity of that loss," she said. "If we don't have to lose these lives then why?"