Making laps around the $1 billion-plus, mostly-taxpayer funded coliseum began at 6 a.m. and lasted two hours. The volunteers represented three conservation groups: Audubon Chapter of Minneapolis, Minnesota Citizens for the Protection of Migratory Birds, and Friends of Roberts Bird Sanctuary.

Their mission was to confirm the worst: To see if the stadium, with its 200,000 square feet of clear and reflective glass, is so indistinguishable to birds that the creatures crash into it like something out of an Alfred Hitchcock movie.

Welcome to the avian killing fields.

Over the course of the monitoring period, volunteers found 60 dead birds. Another 14 were discovered stunned, laying on the ground.

Among the casualties were 21 white-throated sparrows, nine ruby-throated hummingbirds, and one snow bunting, a.k.a., "snowflakes," an uncommon sighting for bird-watchers like Sharpsteen, who says he's never seen anything like this in downtown Minneapolis.

The findings, along with reports from maintenance staff and security guards, estimate that perhaps as many as 500 birds die annually as a result of the building. Even if the actual number is half that estimate, it would still make the stadium the most lethal structure for birds anywhere in Minnesota.

"We knew that the glass would be highly confusing to the birds," Sharpsteen says. "They see a reflection of a blue sky in the glass, they think it's a blue sky. They see reflections of trees, they think they can land in those reflections of trees. This confirmed what we already believed would be bad."

The carnage was especially substantial during the thick of the migration season, that swath of time from late summer into mid-fall when birds by the thousands utilize the Mississippi Flyway, a bird migration route stretching from Canada to South America.

The stadium happens to be located smack dab in the heart of that flight path.

The National Audubon Society has contracted with the Minnesota Sports Facilities Authority and the Vikings to conduct another monitoring study starting this year and continuing into 2018.

To the likes of the Audubon Chapter of Minneapolis' Ann Laughlin, another study will only report what they already know.

"Birds are dying because of the glass on the stadium," she says. "The only thing that a new study does is buy the Vikings time [to not have to do anything]."

That's not good enough, according to Sharpsteen.

"We want them to either replace the glass with a less reflective glass or put a coating on the glass that would make it more bird friendly," he says. "I think the more realistic would be to apply coating to the outside of the glass."

Authority spokesperson Jen Hathaway reserved comment until "the study is finalized in 2019."

Photos courtesy of Audubon Chapter of Minneapolis.