The Audubon Society says much of the blame for the nationwide loss of birds goes to sprawl, agriculture, and energy development.
"The longer term concern is really the habitat destruction," says Greg Butcher, director of bird conservation for the organization.
Human activities reduce grasslands, forests and wetlands and have a huge impact on bird populations, according to Butcher. The study found that 20 common bird species have seen their numbers dwindle by as much as 80 percent over the last four decades.
In Minnesota, Audubon officials say the ongoing loss of grassland and wetland habitats has been catastrophic to some species.
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Among the hardest hit are eastern and western meadowlarks. These birds have been most affected by the loss of grasslands to industrialized agriculture and suburban development.
Minnesota's hardest-hit list also includes purple martins, northern pintails, indigo buntings and red-headed woodpeckers.
Red-headed woodpeckers are down by 89 percent. Some of their habitat has been destroyed by the removal of older and rotten trees in urban areas. They also lose habitat when trees are removed for fire suppression.
Audubon officials say birds have many of the same basic needs as people.
"They need clean water, so do humans. They need food, so do humans. They need clean air, so do humans. So as birds go, so go humans," says Mark Peterson, who directs the Minnesota chapter of the National Audubon Society.
Peterson says birds are often an early warning system for dangerous changes in the environment.
"As some of these populations decline, and we're finding some very steep declines with many of these bird species, that should be a signal to us that something is wrong in the environment. And if those trends continue, certainly that can have impacts on humans as well," Peterson says.
The study is based on 40 years of data collected by scientists, volunteer bird counters and the U.S. Geological Survey. The research found that the area between Highway 95 and the St. Croix River is important for as many as 300 different bird species for migrating, feeding and breeding.
But like other waterfront areas, the St. Croix River Valley has become increasingly valuable real estate. This puts bird habitats at risk of being carved up for development.
Minnesota's population is projected to grow by more than 1 million people over the next 25 years.
Audubon Minnesota's director of bird conservation, Mark Martell, says it's crucial to prevent further loss of habitat in the St. Croix River Valley flyway.
"Keeping it together provides areas that are large enough for birds to find food, find nesting areas, those kinds of things. Most birds have a minimum area they need in order to successfully reproduce," Martell says. "Also, as you get breaking up into smaller pieces, it allows easier access for predators to get in. Most predators are more successful at the edge of a habitat than right in the middle."
Martell says this is especially true in areas near residential development, where many cats and raccoons live.
The threat to wildlife habitat is growing as global warming and population growth increase.
To call attention to the area's significance, the National Audubon Society has identified a 25-mile stretch along the St. Croix, from Stillwater to Taylors Falls, as an "important bird area -- or IBA.
The designation is symbolic, but bird advocates hope it will lead to increased awareness and protection of wildlife habitat.