Collisions with glass kill birds, but what does it do to bird populations?

The impassioned discussion about the amount and size of the glass in the new Minnesota Vikings stadium has already resulted in something positive: Many more people now know that large expanses of glass can be deadly for birds.

At least that's how Robert Zink, a University of Minnesota ornithology professor, and Mark Martell, director of bird conservation for Audubon Minnesota, look at it.

Each year in North America, birds die after trying to fly through window glass. Estimates vary widely from just under 100 million to just under 1 billion.

There is glass that lessens collision risk. It's called fritted glass. It contains visible elements. And it's expensive.

Local birders started a campaign to push for the bird-friendly fritted glass on the new stadium, which would add about a million dollars to the price tag. The Minneapolis City Council approved a resolution calling for use of the more expensive glass. The Metropolitan Sports Facility Authority (MSFA), which controls spending for the building, has said the budget and plans were already set. The Vikings, who could step up with check in hand, have been mum on the subject.

Before construction began, the team had conversations with Audubon Minnesota, and made some concessions to benefit birds, according to Martell.

"The MSFA and the Vikings brought in their architects and construction people to talk with us," he said. "They agreed to turn down lighting during spring and fall migration, as do other buildings downtown. This lessens the chance of collision. We talked about modifying the glass. They said it was a matter of cost."

"It's one-tenth of one percent of the total cost of the building," he said. "We spend a lot of public money to conserve birds. Here, we're spending public money on something that will kill birds."

There's no doubt that glass kills individual birds. But there's a larger question:

Do those deaths ultimately reduce bird populations?

No one knows, said Zink. "The question is not, do birds fly into windows. Of course they do. The question is, does it matter to populations?"

To try to answer that question, Zink and a colleague did a study on bird/glass collisions in 2011. They found that, for many species of songbirds, the deaths of individual birds through collisions with glass have little or no effect on populations from year to year.

"Ovenbirds, for example, hit windows in large numbers, but they are a common species with a relatively stable overall population," he said. "It is not clear that window collisions are influencing ovenbird population numbers."

There are other species that hit windows with high frequencies relative to their overall populations. Why these "super-colliders" experience more frequent strikes is unknown, Zink said. There also are species, blackbirds and swallows for instance, with small collision numbers.

Zink said more research needs to be done on why certain species are more vulnerable to window collisions than others.

He sees the stadium as an opportunity to do that research. In fact, he offered to conduct a study on the effect of glass collisions on bird populations. So far, he hasn't received a response from the Vikings or the MSFA.

While Zink, like most bird experts, realizes that collisions are significant, they aren't the leading cause of population declines.

Author/artist David Sibley agrees.

On his website (www.sibleyguides.com), Sibley lists various causes for individual bird kills. Window strikes come second to habitat loss. Other causes include feral cats, high-tension wires, cars, pesticides and wind turbines.

All of these, he writes, "can certainly have an adverse effect on population size."

But he points out that bird deaths may not harm the population of a given species.

"Studies of hunting have documented that in certain cases killing small numbers of birds can improve the health and survival of the remaining birds. As long as the habitat is intact, [that] population has the potential to replace the lost birds," Sibley wrote.

Surely, the Vikings and the MSFA can work with Zink on such research.

Lifelong birder Jim Williams can be reached at woodduck38@gmail.com. Join his conversation about birds at www.startribune.com/wingnut.