U.S. Bank Stadium officials are going to take some time weighing options to reduce bird collisions at the stadium, ranging from applying patterned film on the stadium's glass surface to removing vegetation around the building.

They received recommendations Friday at a meeting of the Minnesota Sports Facilities Authority (MSFA) that came out of a two-year study on stadium bird fatalities. The study determined that an estimated 111 birds — migrating songbirds, not large birds or city birds like pigeons — die annually from collisions with the building.

MSFA Chairman Michael Vekich said officials need to take in the findings and conduct their own reviews of the cost benefits of various mitigation options.

But the basic plan is to apply film to some portion of the 200,000 square feet of glass on the 270-foot-tall building and to further study the effect of vegetation reflected off the building, which confuses birds when it's planted nearby.

No matter what is done, everyone agrees that birds will continue to die. The big question, according to Vekich: "What's the acceptable collision rate?"

The MSFA, the public body that oversees the stadium; the stadium's main tenant, the Vikings; and stadium operator ASM Global expect to take months before deciding on a solution. Vekich said he expects that a request for proposals will go out no sooner than summer 2020.

He also pledged that a final decision will be made in consultation with the Vikings, who pay rent as well as an annual lump sum to the building's capital improvement fund — now at about $5 million.

Vikings Vice President Lester Bagley attended the 90-minute meeting but did not comment and left without making a public statement.

Several bird conservationists praised the study during the public comment period but also urged the MSFA to take action before the spring migratory season. The overwhelming number of bird deaths occur during spring and fall migrations up and down the Mississippi flyway, the study reported.

"Retrofitting the stadium to protect birds will be the last impediment to making U.S. Bank Stadium a world-class environmentally friendly facility," said the Audubon Chapter of Minneapolis' Jerry Bahls.

Audubon Minnesota executive director Rob Schultz presented the final report, based on the two-year study of bird mortality at the stadium and released Wednesday. He said film on the glass could reduce bird collisions by 80 to 85%.

The 27-page report presented Friday was the final product of the peer-reviewed academic study led by Oklahoma State University associate professor Scott Loss, an internationally recognized bird mortality expert. The Vikings and the MSFA split the $300,000 cost of the study.

Steve Greenfield, president of the Friends of Roberts Bird Sanctuary, a nonprofit support group for the south Minneapolis nature area, encouraged the MSFA to look at durability of the products. He said a patterned film on the glass wouldn't diminish the stadium's open-air feel.

The report provided options and prices that would cover 12,000 to 93,000 square feet of the glass with three specific products applied externally. They range in price, depending on the amount of coverage, from nearly $40,000 to $570,000; those prices don't include labor and installation, which are expected to at least double the cost.

The Oklahoma State-led study found that four downtown Minneapolis buildings accounted for 74% of bird collisions and 68% of fatalities among 21 buildings surveyed. The stadium ranked third among those. Annual fatality estimates at all four buildings ranged from 79 to 216.

Though the numbers strike some as small, conservationists argue that birds migrate to Minnesota to mate and there's a multiplier effect as future generations of birds are lost with each death.

Rochelle Olson • 612-673-1747 Twitter: @rochelleolson