On a postcard-perfect morning in September, construction is underway at Allianz Field in St. Paul, the soon-to-be new home of Minnesota’s professional soccer team. Amid the clanking, beeping, and general hullabaloo, managing partner Bill McGuire is talking about birds. Not the Loons, as fans might expect—after all, that’s the nickname of Minnesota United FC—but rather orioles: The birds are passing through on their annual long-haul journeys south, and McGuire likes to put out protein-packed morsels to help fuel the neotropical migrants’ treks. “I spent the morning getting the mealworms ready, just in case some are still moving through,” he says.

McGuire, a trained medical doctor and former CEO of UnitedHealth Group, has long been captivated by winged creatures. “Monarchs are back—they’re having a good year,” he says, pointing to a flitting burst of orange at my feet that I awkwardly sidestep. “Once you start looking at birds and butterflies, you see the world differently,” he says.

So perhaps it isn’t surprising that since McGuire first began planning the Loons’ new stadium in late 2015, he’s taken birds into consideration, striving to build a structure that pleases the fan base while preventing species like those orioles from crashing into it.

As many as a billion birds die every year in North America from colliding with buildings. Reflections in glass windows create the deadly illusion of sky or vegetation, and birds simply don’t see transparent barriers such as the walls of atria and walkways. Most of the animals that fly full-speed into windows are passerines—perching birds such as orioles and warblers—and most strikes occur during migration. Here in the Twin Cities, hundreds of thousands of migratory birds representing more than 300 species pass through during the spring and fall. In autumn huge concentrations barrel straight down from the boreal forest, pausing to rest and refuel along the Mississippi River, which cuts through St. Paul and Minneapolis. It makes for a perfect storm, says Christine Sheppard, director of the American Bird Conservancy’s Bird Collisions Program: “Birds and water and migration and buildings is always a dangerous combination.”

Bird strikes are a serious problem, but not an intractable one: Over the past quarter century, as awareness of the threat posed by buildings has grown, multiple collision-prevention products have hit the market, most of which are designed to make glass apparent to birds. Increasingly, architects are incorporating bird-friendly elements into their designs, homeowners are marking their windows and sliding-glass doors to make them less deadly, and cities and states—including Minnesota—are instituting bird-friendly building codes.

Allianz Field isn’t receiving any public funding and thus isn’t subject to Minnesota’s code, which went into effect in 2012. But it’s going above and beyond the rules anyway. “They didn’t need an organization or regulations telling them to make a bird-friendly stadium,” Rebeccah Sanders, vice president of the Great Lakes and Upper Mississippi Flyway for Audubon. “They recognize the importance of Minneapolis along the flyway, and they did it on their own.”

A Ruby-throated Hummingbird outside the main gate of U.S. Bank Stadium. Photo: Camilla Cerea/Audubon

These dramatically different approaches reflect the tension around the push for bird-safe buildings.

McGuire’s mission to build a bird-safe stadium stands in stark contrast with the owners of another new professional sports stadium in town. From the time that U.S. Bank Stadium, the $1.1 billion home of the Minnesota Vikings NFL team, was in its design stages, bird experts predicted that the structure would be an avian abattoir; its massive glass facades, they warned, would be indistinguishable to birds’ eyes from the surrounding sky. But the Vikings’ owners were unmoved, and because the team had attained approval for its new home just before the building code took effect, the stadium was exempt from its requirements. U.S. Bank Stadium opened, glass and all, in 2016, just as plans for Allianz Field were getting underway.

These dramatically different approaches reflect the tension around the push for bird-safe buildings. The Vikings stadium, for all its criticism from bird lovers, has been lauded by others as bold and modern, heralded as ushering in a new wave of stadium design that breaks the mold of massive concrete edifices. The Loons stadium, too, has earned accolades for its spectacular design. There’s no longer any question that it’s possible to meld eye-catching architecture with environmental considerations like energy efficiency. The question now is whether we’re willing to redefine what constitutes innovation, and what today’s architects can, and should, consider. Do we cling to an aesthetic that prizes glass-dominated structures? Or embrace a new one that values the lives of birds more?

The exterior of Allianz Field has little glass, but fans will still be able to discern features outside the stadium through the translucent PTFE material that makes up the majority of the facade.

Collision Course

A half-mile from the banks of the Mississippi River in downtown Minneapolis sits U.S. Bank Stadium, its 200,000 square feet of reflective windows forming much of the 270-foot-high structure, creating an illusion of verdant vegetation and a vast swath of sky. Architects drew inspiration for the dramatic lines, cut by the sloped roof and the triangular shards of exterior walls, from the jagged ice formations on nearby St. Anthony’s Fall. The pièce de résistance is the western façade, a massive uninterrupted span of glass that reflects the downtown skyline.

In addition to mirroring the sky, the windows of U.S. Bank Stadium also reflect nearby vegetation. Birds that are foraging may fly into the illusion of trees. Photo: Camilla Cerea/Audubon

The lethal consequences of that huge expanse of seeming habitat were clear on an early September morning. A lap around the stadium before heading inside for a public tour turned up a Nashville Warbler, its bright yellow feathers popping amid the sharp gray rocks at the base of the building, its trek to Mexico cut short. Nearby lay the broken body of a Bay-breasted Warbler that had likely been winging it from the spruce-fir forests of central Canada to South America. And nestled in a crack on the concrete plaza before five stunning 95-foot-high glass doors was a female Ruby-throated Hummingbird, a tiny thing that weighs a scant three grams but has the ability to push 500 miles non-stop across the Gulf of Mexico. As crews inside the stadium prepared for the Vikings’ first home game of the season, outside these three endurance athletes of the avian world stiffened in rigor mortis.

As early as 2013, when renderings of the stadium were first released, bird experts and environmentalists warned that the magnificent structure would kill birds. Despite mounting pressure from Audubon and other groups, and a unanimous city-council vote in 2014 in favor of installing bird-friendly glass, the Vikings and Minnesota Sports Facilities Authority (MSFA), the entity established by the state legislature in 2012 to control and operate the stadium, refused to alter the design. The two organizations said they “didn’t have the budget” to install fritted glass, which has a pattern incorporated during fabrication that enables birds to see it, and which would’ve added $1.1 million to the $1.1 billion price tag, according to the owner’s calculations.

From many of U.S. Bank Stadium's 66,000-plus seats fans can get a view of the downtown Minneapolis skyline. The translucent plastic roof, made of a material called ETFE, allows daylight to pour into the building from above. Photo: Camilla Cerea/Audubon

In addition to the cost, there was also the question of aesthetics. Applying a film to the glass, or a design fritted into it, would clash with the “design goals,” Michele Kelm-Helgen, then chair of the MSFA, explained at the time. The stadium would, Kelm-Helgen said, participate in Audubon’s Lights Out program, flipping the switch during migration. (Lights can disorient birds traveling at night, causing them to crash into buildings or circle in a spotlight beam until they collapse from exhaustion.)

“It’s perplexing and sad that they didn’t treat the issue as something that should be of concern,” says Daniel Klem, an ornithologist at Muhlenberg College in Allentown, Pennsylvania. Klem is a collision expert who has tested bird-strike products for decades. “As beautiful as the stadium is for perhaps the majority of people, it loses its luster when you realize that it’s killing these innocent and useful animals that so many others care so much about,” he says.

The refusal was all the more confounding to bird-strike experts because the Vikings took pains to ensure that the structure incorporated enough eco-friendly elements, including using recycled concrete from the old stadium and LED lighting, to earn a LEED Gold rating from the U.S. Green Building Council.

U.S. Bank Stadium could have earned the GBC’s Bird Collision Deterrence credit, created in 2009, if, among other measures, it had employed fritted glass, which is both visible to birds and more energy efficient than traditional glass—an appealing win-win for many architects seeking certification, says Sheppard. In October, Fiserv Forum, new home of the Milwaukee Bucks basketball team, became the first sports arena to earn the bird-friendly credit. The pending LEED Silver design incorporated bird-safe glass, and the Bucks will take part in Lights Out as well as undertake a three-year collision monitoring effort, per the guidelines.

As for the aesthetic concerns, Sheppard argues that there are multiple options that would largely preserve it, such as glass with a narrow horizontal stripe every two inches that isn’t visible to humans from a distance of around 20 feet. “If the Vikings had used that type of glass on those giant doors, I’m fairly sure that people in their seats couldn’t perceive that, whereas a bird that approached the glass would see it and turn away,” she says.

It is possible to retrofit buildings to reduce risk. After the windows of New York City’s Javits Center were replaced by bird-safe glass in 2015, for example, avian deaths fell by 90 percent. A similar retrofit at U.S. Bank Stadium would cost an estimated $10 million—$9 million more than installing it in the first place. For fans, as Sheppard points out, the change would likely have little, if any, affect on the scenery, inside the stadium or out.

By mid-afternoon on the recent September visit to U.S. Bank Stadium, a group of tourists arriving for a tour passed within a few feet of the hummingbird carcass on the concrete. None noticed it; they were looking up, marveling at the skyline reflected in the glass façade.

The Vikings didn’t seem to care if the decision resulted in a PR fumble.

The Vikings didn't seem to care if the decision resulted in a PR fumble. Which it did. Headlines in national and international news outlets have since called the Vikings’ glistening new home a “death trap for birds.”

The week before the stadium opened, in July 2016, the Vikings and MSFA agreed to fund a three-year, $300,000 study to monitor bird-window collisions. The study, undertaken by Audubon Minnesota, National Audubon Society, University of Minnesota, and Oklahoma State University, launched in the spring of 2017 and wraps up this fall. “If we identify that there is a problem, we can partner with [glass company] 3M to provide a bird safe solution,” MSFA’s Kelm-Helgen said when the study was announced. (MSFA and the Vikings didn’t respond to requests for comment for this story.)

Safety By Design

Five miles away in St. Paul’s Midway neighborhood, the Loon’s new home draws the eye not up, but across the low-profile, open-air oval. This morning 150 or so workers are on site. It takes only a handful of them—plus a couple of lifts—30 minutes to hang a gleaming 11,000-square-foot expanse of silvery fabric, one of 10 ginormous panels that will wrap around the metal frame of the 20,000-seat, $250-million stadium. The undulating design, McGuire tells me, also takes its cue from the environment: It is meant to evoke the Mississippi River and the state’s innumerable lakes.

In a nod to one of the state’s nicknames, the fritted-glass doors soon to be installed will feature etchings of the North Star, the emblems spaced at most 2 inches apart, a span small enough that songbirds won’t try to slip through, Klem’s research shows. The glow from 5,000 linear feet of LED lights encircling the stadium—instead of bright spotlights—will minimize light pollution and the risk of disorienting night migrants, and echo the Northern Lights that dance across the night sky. “Nature and natural resources are a big part of Minnesotan’s heritage, so it’s important to remember those elements,” McGuire says. “They’re important to the people here.”

McGuire sees preventing collisions as a natural extension of those values. “From the start we wanted to marry the design with the bird issue,” he says. McGuire, architects from international firm Populous, and other partners on the project began by poring over a bird-friendly building design guide created by Audubon Society of Portland to understand best practices and the products available. They also reached out to Audubon when they began considering the design and material options and have continued to consult with the organization and other bird experts throughout the process. “I think people often think that bird-friendly design is going to be boring,” says Audubon’s Sanders. “It doesn’t have to be boring—it can be amazing.”

Ultimately, they settled on a material that had never been used before and allowed them significant flexibility with the design. The 90,000-square-foot sleek skin of the stadium is comprised of dyed, laminated PTFE mesh, a lightweight, flexible material reminiscent of patio furniture—only far stronger and more durable. The woven, translucent material, made by French company Saint-Gobain, lets light in while keeping wind and other elements out—something project manager Greg Huber especially appreciates. (“It’s pretty, but the weatherization aspect is my favorite part,” he says.) Engineers put the material, which they estimate has a 30-year lifespan, through a year and a half of rigorous testing, ensuring that it can withstand temperatures ranging from -30 to 110 degrees Fahrenheit. At about $50 per square foot the material isn’t cheap, but it’s still only about half the cost of glass.

The exterior of Allianz Field is wrapped in polytetrafluoroethylene (PTFE), a versatile material with a wide variety of applications. Photo: Camilla Cerea/Audubon

As we follow the internal walkway that rings the stadium, diffuse natural light pours through the PTFE. The 10 huge translucent panels are comprised of numerous smaller ones sewn into a handsome herringbone pattern, through which the shapes of trees planted along the exterior of the building are visible.

The vegetation is intended to draw birds to the grounds, but in a safe way. Instead of spindly saplings, McGuire opted for nearly 200 native trees that are 8 to 11 years old—ready-made avian habitat. To the north of the stadium, a 1.5-acre green space will sit atop a storm-water system that will harness and re-use rain and snowmelt for irrigation, minimizing discharge. The structure is also sunk 18 feet beneath the surface, making it less of an obstacle for birds—and better melding it into the surrounding neighborhood. The goal of all these carefully chosen design elements is simple: Fans will flock to Allianz Field, McGuire hopes, while birds keep a safe distance.

Spring Training

Come this spring, both the Loons’ and the Vikings’ new stadiums will be tested.

By then, or soon after, the Vikings will likely have the results of the bird-window collision study at U.S. Bank Stadium. Football season will have ended, but the stadium will be preparing to host a Men’s Final Four basketball game. To comply with NCAA guidelines that no natural light can stream onto the court, the Vikings may spend as much as $5.2 million to install curtains on the interior side of the windows. To Klem and Sheppard, it’s a worrisome solution, as it will likely increase the reflectivity of the glass from the outside, possibly making the building even more dangerous for birds.

Buildings are just one of the threats that migrants encounter as they cover hundreds to thousands of miles twice a year. They also have to contend with power lines, oil pits, cats, habitat degradation, storms, and other obstacles. Considering birds in the blueprints, as McGuire has done, and retrofitting existing structures, as the Vikings have indicated they might do, is a significant step in helping ensure safer passage on their incredible journeys.

In April, McGuire will be watching for the orioles and other migrants to return and anticipating the Loons' first match at Allianz Field. He will have an opportunity to see how both soccer fans and the birds passing through on their way to their northern breeding grounds respond to the new stadium. While nobody expects collisions to occur, McGuire says they’ll be keeping an eye out. “If there’s one dead bird,” he says, “I’ll hear about it.”

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