Bird population researchers see a troubling trend: North America has nearly 3 billion fewer wild birds than in 1970. A comprehensive new study shows a drop in sheer numbers of birds, not extinctions. The U.S. and Canadian bird population was probably around 10.1 billion nearly half a century ago and has fallen 29% to about 7.2 billion birds, according to last week's report in the journal Science. Cornell University scientists and others projected population data using weather radar that captures migrating flocks, 13 surveys going back to 1970 and computer models to come up with trends for 529 species of U.S. and Canadian birds.

Habitat loss is the top reason for loss of common birds such as sparrows, orioles, thrushes and bluejays, experts say. Other factors are cats, window collisions and vehicles. Here's why it matters, according to Margaret Rubega, an associate professor of ecology and evolutionary biology at the University of Connecticut: "Three billion of our neighbors, the ones who eat the bugs that destroy our food plants and carry diseases like equine encephalitis, are gone. I think we all ought to think that's threatening." In addition, others note that birds reflect environmental health. They see mass declines as signs of a planet in trouble. "The loss of nearly three billion birds signals a looming crisis that we have the power to stop. We call on all our lawmakers, political candidates and voters across the continent to place renewed value on protecting our common home," two directors of institutes behind the new report write in The New York Times last week.

Sara Hallager, bird curator at the Smithsonian Institution in Washington, shares steps that can help: Keep cats indoors, treat home windows with tinting or other materials to reduce the likelihood that birds will fly into them, stop pesticide and insecticide use in yards and gardens. "We can reverse that trend," Hallager says. "We can turn the tide." There's also this bright note: "The numbers of ducks and geese are larger than they've ever been, and that's not an accident," says study leader Kenneth Rosenberg of Cornell University. "It's because hunters who primarily want to see healthy waterfowl populations for recreational hunting have raised their voices."

Lead researcher says: "One of the scary things about the results is that it is happening right under our eyes. We might not even notice it until it's too late." -- Kenneth Rosenberg, Cornell University conservation scientist in Ithaca, N.Y.

Museum expert says: "This put numbers to everyone's fears about what's going. It's even more stark than what many of us might have guessed." – Joel Cracraft, ornithology curator at the American Museum of Natural History in New York City

Environmentalist says: "This staggering loss suggests the very fabric of North America's ecosystem is unraveling. . . . Trouble for birds means trouble for us as well." -- Peter P. Marra, director of the Georgetown Environment Initiative in Washington, D.C.