Cook County shows biggest population decline in U.S.

Cook County lost the most population of any county in the nation, according to new census estimates released Thursday for the period of July 1, 2014, to July 1, 2015.

The drop of 10,488 people is the county's first decline since 2007 and comes in a year when most metro areas in the U.S. gained population.

Illinois hasn't grown as fast as other states in recent years, leading to a loss of seats in Congress. And the new numbers will likely be used in the ongoing political debate over Illinois' job climate.

"We are not creating enough jobs, we continue to lose critical middle-class manufacturing jobs, and unemployment in Illinois continues to rise, keeping our state from achieving full economic recovery," Illinois Department of Commerce Director Jim Schultz said in a statement.

The new U.S. Census Bureau data shows slight population declines in Lake and DuPage counties as well, but Kane, Kendall, McHenry and Will counties all saw a bump upward.

Overall, the Chicago metropolitan area - which includes the city, suburbs and parts of southern Wisconsin and northwest Indiana, shows a population drop of almost 6,300 residents. The number of births and new residents didn't offset the loss.

Experts suggest the reasons for decline include a drop in immigration, an aging Mexican immigrant population and dwindling numbers of black residents.

Chicago demographer Rob Paral told the Chicago Sun-Times that the population drop really is no big deal, given the region's population of about 5.2 million in Cook County and 9.5 million in the metropolitan area.

"When you hear that Cook County lost the most people in the country, well, it's also the second-largest county in the country," said Paral, who works with nonprofit groups in the city.

The region's net population figure has generally been defined by declines in population in Chicago, buoyed by growth in suburban Cook and the collar counties, Paral told the Sun-Times. Development in collar counties has slowed as communities become "built up," Paral said, allowing room for fewer new residents. Older people have fewer children.

The Chicago metro area remains the third-largest nationwide, the census report showed. Wayne County, Michigan, which includes Detroit, had the next-biggest decline, with 6,673 people lost.

The biggest growth in a metro area not in the south or west: Fargo, North Dakota, which gained 5,279, according to the Bismarck Tribune.

• The Associated Press contributed to this story.