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Much concern has been expressed about potential bird deaths from crashing into the glassy U.S. Bank Stadium, but it's cats — yes, common felines — that pose a much bigger threat.

While federal wildlife officials estimate that up to 1.6 million birds in the United States are killed yearly after striking high-rise buildings, by far, the No. 1 killer of wild birds is cats, both domestic pets and feral, free-ranging animals.

A study published in 2015 said cats kill an estimated 1.3 billion to 4 billion birds annually in the United States.

That's billion, with a B.

"I think people would be rightly shocked," if they knew the extent that cats prey on birds, said Tom Will, U.S. Fish and Wildlife Service regional migratory bird coordinator based in the Twin Cities. He is co-author of the study that examined human-caused bird mortality.

The findings show that we are making it a very perilous world for birds. Though bird deaths from high-rise buildings often get much publicity, they are a fraction of those caused by other sources, including automobiles, power lines and communication towers.

And cats.

The numbers of birds killed by felines may not be surprising considering there are an estimated 86 million pet cats, and perhaps 60 million or more feral cats roaming the nation.

The study estimated feral cats kill between 800 million and 3 billion birds, while domestic cats allowed to wander outdoors kill between 221 million and 1.7 billion.

The solution is obvious: Don't let pet cats roam free outdoors, and reduce the number of feral cats, Will said. In the wild, cats are essentially an invasive species and effective predators.

But the issue is highly controversial and emotional, with no easy solutions. Some cat advocates support trap-neuter-release programs, in which feral cats are trapped, neutered, and then released. The theory is that over time their numbers will fall.

"There's been no substantiation of that hypothesis," Will said. "In fact, in studies that have been done, the opposite occurs: more cats turn up because the policy usually is accompanied by feeding stations."

The authors of a book from last fall, "Cat Wars: The Deadly Consequences of a Cuddly Killer" (Princeton University Press, $24.95), say the most desirable solution would be to remove all free-ranging cats from the landscape "by any means necessary."

The book by Peter Marra, co-author of the bird mortality study and head of the Smithsonian Migratory Bird Center, and writer Chris Santella, estimated there could be 60 million to 100 million free-ranging and unowned cats in the country. They say cats have caused a minimum of 33 global extinctions and significant declines in another 142 bird, reptile and mammal species.

Their book has been criticized by the Humane Society of the United States, which agreed there are too many wild cats, but supports trapping, neutering and releasing cats rather than euthanizing them.

But the Humane Society does recommend cat owners keep their cats indoors.

"Predation by outdoor cats on birds and other wildlife is a real and legitimate concern," the group states on its website.

Cat owners too often let their cats roam neighborhoods, wildlife officials say. And some people may simply be willing to sacrifice wildlife for felines. Others who help feed feral cats may not realize the potential damage the cats do to wildlife.

"People have a relationship with cats that they don't have with wildlife," Will said.

Other bird killers

Hitting buildings are the second-largest human-caused bird mortality factor, resulting in an estimated 365 million to 988 million birds annually. Most vulnerable are migratory songbirds such as warblers, vireos, thrushes and sparrows.

High-rise buildings like skyscrapers are responsible for up to 1.6 million birds annually, while low-rise buildings are responsible for up to an estimated 715 million bird deaths.

But people only have to look out their windows to see another major source of bird mortality. Residences account for up to 378 million bird deaths a year — or almost a third of all building-related bird mortality.

"On a per-building basis, these big skyscrapers with lots of glass kill more birds than an individual home," Will said. And, like the new Vikings stadium, those glass towers tend to attract more concern because they are so visible.

"But there are so many more individual homes," Will said. And because of those sheer numbers, they are responsible for far more bird mortalities than skyscrapers.

Special glass that discourages bird collisions is available for tall buildings. It was rejected for use on the new Vikings stadium, though the stadium's effects on birds is being studied.

Homeowners have some solutions available.

"People can take actions to reduce window-collision kills," Will said.

Audubon Minnesota, for example, recommends placing bird feeders and birdbaths either within 3 feet of a window, or farther than 30 feet away.

And tape or decals on exterior of windows can be effective to break up reflections.

But there are many other human-caused contributors to bird mortality, the study reported:

• Automobiles kill an estimated 89 million to 340 million birds a year. Barn owls are particularly susceptible, and bald eagles have increasingly become victims as their population has rebounded.

• Power lines are responsible for up to 69 million bird deaths annually, both from strikes and electrocution. Some practices have been developed to reduce bird deaths, including spacing of wires and marking them to make them more visible to birds.

• Communication towers are blamed for an estimated 6.5 million bird deaths annually. Again, migratory birds are most vulnerable. Birds are attracted to tower lights during night migration.

"Studies have shown that you can reduce mortality by 70 percent by switching from steady lights to strobe lights," Will said. "And it's a win-win, because those lights are less costly and easier to maintain."

• Wind turbines are responsible for 460,000 to 680,000 bird deaths each year, but the number of turbines has been increasing as wind energy expands and that expansion is expected to continue.

• The widespread use of agricultural pesticides is an unknown factor in the U.S. because of a lack of studies, but Canadian studies have estimated bird deaths from pesticides there at from 1 million to 4.4 million. Given the intensive agriculture in the U.S., pesticides could be causing significant bird mortality.

What about habitat?

Habitat loss and climate change also are factors in declining bird numbers. In the U.S., 100 bird species and subspecies are listed as federally threatened or endangered, and without conservation action another 200 species could be added, the study concluded.

But it's unclear what impact the human-caused bird mortality is having on individual bird populations.

"We don't know," Will said.

"Most biologists would agree that habitat loss is the biggest reason we have declines in bird populations. We are losing wetlands and grasslands. Then you add in all these other things on top of that. You keep stacking the deck with obstacles, and sooner or later you definitely will have a population effect."

Bird populations have declined over the past 50 years. The North American Bird Conservation Initiative reported last spring that one-third of North American bird species need urgent conservation action.

And Partners in Flight, representing 150 organizations in the Western Hemisphere, reported that breeding land-bird populations in the continental U.S. and Canada have fallen by 1 billion since 1970, including 22 species that have lost at least half of their population.

Said Will: "We're not making it easy for birds and other wildlife."

Doug Smith is a retired Star Tribune outdoors writer. Reach him at