Ask any bird expert what the leading causes of unnecessary bird deaths are in the U.S. and they will probably say domestic and feral cats and buildings.

The folks planning the new $900 million Essentia Health Vision Northland campus in Duluth now under construction can’t do much about the cats. But they did pick a type of glass for the new buildings that may reduce how many birds hit the windows and die.

With Duluth a major bird migration route, as birds skirt around Lake Superior each spring and fall, Essentia is using “fritted” glass in the design of its 15-story hospital tower and eight-story clinic tower to avoid bird strikes. Frit — a ceramic layer with lines, dots or other patterns — breaks up the reflectivity of the glass and makes it visible to birds. Birds recognize that there is a solid object in front of them and avoid it.

Beyond conforming to guidelines from Audubon Minnesota, much of the frit pattern also meets the stricter Ontario requirements, which stipulate that the frit openings should be 2 inches by 4 inches. Crews have been busy installing the panels, which are nearly 5 feet wide and 17 feet tall, since Jan. 20 and will continue for the next year.

The glass, manufactured by Viracon Glass in Owatonna, Minnesota, consists of two panes. The frit will go on the inside surface of the outside pane. Essentia also solicited advice from birding experts in designing evening and night lighting in a way that not only will reduce bird collisions but also limit light pollution.

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“As a Duluthian who cares deeply about birds, and as a longtime client at Essentia, I am delighted with Essentia’s good-faith efforts to make its new facility as safe for migrating and local birds as possible,” Laura Erickson, Duluth-based birding expert, said in a statement. “No glass is truly bird-proof. But selecting the least reflective glass possible and using a dense frit pattern will protect migrants.

"And setting lights-off as the default in public spaces will help prevent nighttime collisions as nocturnal migrants flood through in spring and fall — they get disoriented by lights, especially during the foggy conditions Duluth gets in abundance during migration," Erickson said.

Smithsonian Institution researchers in 2014 concluded that collisions with buildings kill an estimated 600 million birds each year. Of that total, about 250 million are collisions with homes and other buildings one to three stories tall. Larger, low-rise buildings four to 10 stores tall caused 340 million deaths. High-rise buildings, 11 floors and higher, kill about 500,000 birds annually.

Individual skyscrapers can be quite deadly for birds, but they kill fewer birds overall due to their limited numbers.