The soaring, four-acre glass exterior of the new Vikings stadium has for years made birders fearful that migrating birds will be confused by the windows' reflections of the sky or the lights inside the new stadium.
"We estimated that up to 1,000 birds could be killed each year, and that's the worst case scenario," said Jerry Bahls, Jerry Bahls, president of the Minneapolis chapter of the National Audubon Society.
The Vikings and the state have agreed to fund a $300,000 study to monitor bird collisions at the new stadium. But the plan doesn't commit to altering the massive glass panels on the building if the research finds they are dangerous to birds, and some bird advocates say the agreement doesn't go far enough.
Bahls wants the Vikings to make changes to U.S. Bank Stadium to make it easier for birds to see its glass panels.
But the Minnesota Sports Facilities Authority and the Vikings say it's premature to change the windows before it's clear that they pose a risk to birds. MSFA chair Michele Kelm-Helgen says there's no sign that the building has caused any problems so far.
"The glass has actually been on the building, including the west wall of the stadium, for over a year," she said, "and I will tell you that Mortenson's construction team tells us that arriving early every morning, 5 to 5:30 as they come to the site, anecdotally, they have not found a problem with injured birds."
Still, her agency and the Vikings are paying for a three-year bird collision study.
The money is going to the state chapter of the Audubon Society to fund four scientists, whose report is due in 2019.
The Audubon Society is wrapping up its own a decade-long study of bird collisions. The researchers walked mapped routes in downtown Minneapolis and St. Paul to track bird deaths involving buildings.
Joanna Eckles, the bird-friendly communities manager with the Minnesota Audubon Society, says it would ultimately be best to address bird strike issues before buildings go up.
"If you leave it up to people's discretion, it gets value-engineered out so often, or it doesn't get included in the whole plan for a building," Eckles said.
Experts say it's a legitimate problem.
"Based on a scientific review of existing literature and existing data from across the United States, we have actually come up with a peer reviewed and published estimate that somewhere in the hundreds of millions of birds are dying in the U.S. each year by colliding with buildings," said Scott Loss, an assistant professor at Oklahoma State University who studies migratory bird mortality.
Loss will be one of the four experts working on the U.S. Bank Stadium study. He says they'll compare bird strikes at the facility to other buildings in the area.
That said, there isn't a threshold for bird deaths that will trigger a change at the stadium, which upsets bird advocates who wanted a guarantee that something would be done.
Bahls said the Minneapolis chapter of the Audubon Society, separate from the state group, may consider legal action against the stadium.
The Sports Facilities Authority says Maplewood-based 3M may have a film that will keep the stadium glass transparent and also be visible to birds.
But the agency wants to assess the problem before it considers taking any steps.