The Minnesota Audubon Society say the Minnesota Vikings' proposed glass stadium will be a "death trap" for birds and is calling for design changes in the stadium set to host the 2018 Super Bowl.

The society says Viking management and the Minnesota Sports Facilities Authority have so far refused its call for safer types of glass and other changes that could save birds migrating along the Mississippi corridor from fatal collisions with the stadium's huge class windows.

"We're talking about a billion dollar stadium here, and the cost to save perhaps thousands of migratory birds  -- and make the Vikings a global leader in green stadium design -- is about one-tenth of one percent of that," said Audubon Minnesota Executive Director Matthew Anderson.

Close to a billion birds die in the United States each year after colliding with buildings and especially with their glass windows, scientists with Smithsonian Institution and the U.S. Fish and Wildlife Service say.

Nearly 200,000 square feet of glass will be used in the new Viking stadium, the design of which includes the world's biggest glass swinging doors.

A request for the facility authority to consider using a special glazing technique that could help avoid or at least reduce bird strikes was refused, the society said, with the authority saying it could not afford the extra $1.1 million the special glass would cost.

The glass could cut down on reflections, the society said, noting that birds aren't able to understand the concept of reflection and will attempt to fly into a landscape that they see, colliding with the glass with often fatal results.

The problem is especially acute around tall, well-lit buildings in poor weather or at night, it said.

Despite a number of meetings with the Vikings and the MSFA, in the end the suggestion about the special glass was turned down, the society said.

The Vikings have released a statement in which they addressed the society's concerns.

"The Vikings and the MSFA have agreed to implement lighting operational procedures, when possible, to mitigate bird collisions during peak migratory periods," the team's statement said. "We are also discussing the Audubon's various recommendations regarding the lighting design for the stadium."

Financial considerations led to the decision about the glass, the team said.

"Stadium construction costs continue to rise, and unfortunately the budget does not include the additional cost required by the Audubon Society's recommended glass. We will continue to partner with the MSFA to enhance bird safety through the lighting design and operational guidelines."