Some of America's most common birds have been declining at drastic rates over the past 40 years due to habitat loss and climate change, according to a study released Thursday by the National Audubon Society.

The study combined data from Audubon's Christmas bird counts and breeding bird surveys and identified 20 common birds with the overall greatest declines in the continental United States.

Each of the lower 48 states also listed five species with the biggest declines.

In Montana, water is the common thread to the five declining species. The American redstart, belted kingfisher, killdeer, blue-winged teal and great blue heron rely on wetlands or streamside forests, according to Dan Casey of the American Bird Conservancy.

The American redstart, a colorful warbler, and the belted kingfisher posted the steepest declines of all Montana birds, dropping 88 percent since 1967.

Great blue herons are down 81 percent; the killdeer, a shorebird that often inhabits urban parks and ponds, has fallen 80 percent; the blue-winged teal, a small duck, has declined 72 percent.

According to Steve Hoffman, executive director of Montana Audubon, a 20-year drought throughout the state has been a major factor behind the declines of all five birds on the Montana list.

"There is strong belief that much of the drought may be due to global warming," Hoffman said. "We should all take steps to reduce that. It's going to affect all wildlife and our own quality of life."

Hoffman said the American redstart depends on lush understory in streamside forests. If that habitat is disturbed by overgrazing, it can affect breeding success.

The kingfisher, Hoffman said, likes calm, clear waters. He said clearing of vegetation could be a factor in that species' decline.

Herons have been affected by a long-term decline in cottonwood forests along rivers, where they nest exclusively.

Teals could be affected by draining of marginal wetlands or farmers plowing to the edge of wetlands.

Hoffman said that the Farm Bill before Congress could have far-reaching impacts on birds by limiting the amount of land that can be plowed.

"What these counts are telling us is there's a problem," Carol Browner, Audubon chairwoman and former Environmental Protection Agency administrator, said in an Audubon teleconference Thursday. "It's not a crisis yet. We may find ourselves in a situation where these birds we know may not be a part of our future."

"The time to save a species is when it's still common," added Scott Weidensaul, a Pulitzer Prize finalist for his book "Living on the Wind: Across the Hemisphere with Migratory Birds."

"What we really need is the will and determination to do it."

Weidensaul said everyone can help protect bird habitat through community activism, donating to groups that protect grasslands or wetlands or just devoting space in their own yards to native plants that provide habitat and food.

"Planting native plants in a corner of your yard can be a big help," Weidensaul said. "Taking what has become sterile backyards and putting a corner of them into native plants is going to make a big difference."

Montana's state bird, the western meadowlark, wasn't on the national top-20 list of declining birds, although it almost made the list, according to panelists in Thursday's teleconference. It is on many state top-five lists, although not Montana's.

According to Hoffman, the western meadowlark dropped 25 percent in Montana's breeding bird survey and 87 percent in the Christmas bird count.

The disparity in the birds' drop in winter numbers to breeding season numbers could be due to farming practices.

"If there's more cover in fields after harvest that will benefit western meadowlarks," he said. "For them to make it through the winter they have to have tall grass cover, and we may be losing that cover."

According to Hoffman, 88 percent of all Montana bird species breed in, nest at or use wetlands or streamside forests at some point each year.

Hoffman said amidst the bad news there is a positive story: Bald eagles, once at the brink of extinction, have increased and continue to thrive. Wild turkeys are back in areas where they'd been long gone, and Canada goose and osprey numbers are also doing well.

"When we decide we want to make a difference, we can make a difference really quickly," Hoffman said.

He added that PPL Montana has put a lot of money into restoring streamside forests in recent years, and since that time, there has been a lot of recovery of some of those streamside birds.