St. Clair County lost the second-most residents of any county in the state last year, and the losses would have been higher without an influx of more than 500 international immigrants, according to the U.S. Census Bureau.

Neighboring Madison County faced significant losses as well.

The population in St. Clair County dropped by more than 1,200 people during 2012, the census bureau reported Thursday. Winnebago County along the Wisconsin border is the only county to lose more people (1,600 residents) than St. Clair in the past year.

Likewise, the population dropped by more than 600 people in Madison County last year -- the fourth greatest loss of residents in the state.

The losses came at a time when both counties saw births outnumber deaths by more than 1,000 babies, and an injection of 527 international immigrants in St. Clair County and 177 immigrants in Madison County.

A Belleville News-Democrat analysis of the census bureau data shows the losses stem from thousands of residents leaving the counties. For example, 2,857 such residents left St. Clair County and 1,249 moved out of Madison County in 2012.

"The brakes that were put on migration during the Great Recession appear to be easing up," said William H. Frey, a demographer at the Brookings Institution who analyzed the migration data. "Native migrants are becoming more 'footloose' -- following the geographic ups and downs of the labor market -- than are immigrants, who have tended to locate in established ethnic communities in big cities.

"Immigration levels are not where they were a decade ago, but their recent uptick demonstrates the important safety valve they can be for areas with stagnating populations," he said.

Both counties' populations have dropped since 2010 -- about 1,400 fewer people in Madison County and 1,200 fewer in St. Clair County. The bureau estimates 267,883 people currently live in Madison County and 268,868 in St. Clair County.

By comparison, Monroe County's population increased by 400 residents to 33,357 people. In Missouri, St. Louis County added 1,484 residents for a total of 1,04 million and St. Louis City lost 1,122 residents for a new estimate of 318,172 people.

One expert believes population growth in metro areas may be hampered by efforts to cut future federal spending, including funding related to Scott Air Force Base.

Mark Mather, an associate vice president at the Population Reference Bureau, noted that plans to downsize government and reduce federal spending also could have a significant impact on future population winners and losers. The bureau is a Washington D.C.-based research organization.

Since 2010, many of the fastest-growing U.S. metro areas have also been those that historically received a lot of federal dollars, including Fort Stewart, Ga., Jacksonville, N.C., Crestview, Fla., and Charleston-North Charleston, S.C., all home to military bases. Per-capita federal spending rose from about $5,300 among the fastest-growing metros from 2000 to 2010, to about $8,200 among the fastest-growing metros from 2011 to 2012.

"Federal funding has helped many cities weather the decline in private sector jobs," Mather said.

The Associated Press contributed to this article. Contact reporter Daniel Kelley at dkelley@bnd.com or 618-239-2501.