Without immigrants pouring into the nation's big metro areas, places such as New York, Los Angeles and Boston would be shrinking as native-born Americans move farther out.

Many smaller areas, including Battle Creek, Mich., Ames, Iowa, and Corvallis, Ore., would shrink as well, according to population estimates to be released Thursday by the Census Bureau.

"Immigrants are filling the void as domestic migrants are seeking opportunities in other places," said Mark Mather, a demographer at the Population Reference Bureau, a private research organization.

Immigrants long have flocked to major metropolitan areas and helped them grow. But increasingly, native-born Americans are moving from those areas and leaving immigrants to provide the only source of growth.

The New York metro area, which includes the suburbs, added 1 million immigrants from 2000 to 2006. Without those immigrants, the region would have lost nearly 600,000 people.

Without immigration, the Los Angeles metro area would have lost more than 200,000, the San Francisco area would have lost 188,000 and the Boston area would have lost 101,000.

The Census Bureau estimates annual population totals as of July 1, using local records of births and deaths, Internal Revenue Service records of people moving within the United States and census statistics on immigrants. The estimates released Thursday were for metropolitan areas, which generally include cities and their surrounding suburbs.

Among the findings:

—Atlanta added more people than any other metro area from 2000 to 2006. The Atlanta area, which includes Sandy Springs and Marietta, Ga., added 890,000 people, putting its population at about 5.1 million. Gaining the most after Atlanta were Dallas-Fort Worth, Houston, Phoenix and Riverside, Calif.

—On a percentage basis, St. George in southwest Utah was the fastest growing metro area from 2000 to 2006. St. George's population jumped by 40 percent, to 126,000. The next highest percentage increases were in Greeley, Colo., Cape Coral, Fla., Bend, Ore., and Las Vegas.

—The New Orleans area, still recovering from Hurricane Katrina, lost nearly 290,000 people from 2005 to 2006, reducing its population to just over 1 million. The Gulfport-Biloxi area in Mississippi, also hit hard by Katrina, lost nearly 27,000 people, dropping its population to 227,900.

—Parts of the Rust Belt also had large declines. The Pittsburgh metro area led the way, losing 60,000 people from 2000 to 2006. Its population loss was followed by declines in Cleveland, Buffalo, N.Y., Youngstown, Ohio, and Scranton, Pa.

—Houston edged past Miami to become the sixth largest metro area, with about 5.5 million people. Miami slipped to seventh.

There are about 36 million immigrants in the U.S. About one-third are in the country illegally. The Census Bureau, however, does not distinguish between legal and illegal immigrants.

The White House floated a plan last month that would grant work visas to illegal immigrants, but they would have to return home and pay hefty fines to become legal U.S. residents.

Lawmakers were unable to reach an agreement last year on how best to stem the flow of illegal immigrants. Immigration was a contentious issue in many congressional races in November.

Many demographers associate shrinking populations with economic problems, typically poor job markets or prohibitive housing prices.

"A lot of cities rely on immigration to prop up their housing market and prop up their economies," said William Frey, a demographer at the Brookings Institution, a Washington think tank.

Advocates for stricter immigration laws question whether a stable, or even a shrinking population, is bad.

"Don't we have concerns about congestion and sprawl and pollution?" asked Steven A. Camarota, director of research at the Center for Immigration Studies, which advocates for stricter immigration policies.

"Maybe those metro areas should think about what it would take to make Americans want to live there," Camarota said.