US Bank Stadium, home to NFL American football franchise, the Minnesota Vikings, has been dubbed a “death trap” for birds with its large, clear panes of glass, which are indistinguishable from the sky.

Over an 11-week period in the autumn, bird enthusiasts undertook regular circuits of the Minneapolis stadium, and discovered 60 dead birds, and another 14 stunned from flying into the glass, according to UK newspaper the Guardian.

The report said that would project to at least 360 deaths over a three-year period, but that number “significantly underestimates true mortality at the stadium complex, because it does not include birds removed by maintenance staff, security guards, and scavengers.”

Jim Sharpsteen, a volunteer who helped to conduct the stadium study, told CityPages: “We knew that the glass would be highly confusing to the birds. 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 stadium, which opened last June and cost more than $1bn (€950m/£808m) of mostly taxpayer money, has proved to be the “death trap” that the Audubon Society predicted back in 2014. The society pushed authorities to introduce changes so the birds could distinguish between the stadium and the sky. “We know the people of Minnesota do not want their money killing birds,” the society said.

However, the Minnesota Sports Facilities Authority, which oversees the running of the stadium, has declined to take any steps, such as installing glass with a visible pattern.