Chicago, Ill. (Wikimedia Commons)

(CNSNews.com) - The counties containing Chicago, Detroit and the independent city of Baltimore were the biggest population losers in the United States from 2015 to 2016, according to data released today by the Census Bureau.

Cook County, Ill., where Chicago is the county seat, had the largest population loss of any county in the country from 2015 and 2016.

Between July 1, 2015 and July 1, 2016, Cook County lost a net of 21,324 people to hit 5,203,499, according to the Census Bureau.

Even while it was experiencing the largest net population loss of any county in the country, Cook Count was seeing an influx of migrants from foreign countries and more births than deaths.

During the year, births outnumbered deaths in Cook County by 68,049 to 42,297, and 18,434 people moved into the country from foreign nations.

But at the same time, 66,244 residents left Cook County for other places in the United States.

Wayne County, where Detroit is the county seat, had the second largest population loss of any county in the nation; and Baltimore, Md., an independent city that the Census Bureau counts as a county, had the third largest population loss.

The general patterns in these counties were the same as in Cook County.

Wayne County lost a net of 7,696 people during the year, ending up with a population of 1,749,366. Births outnumbered deaths in the county 23,209 to 18,231, and 4,279 people moved into the county from foreign countries.

But at the same time, 17,346 people moved out of Wayne County to other places in the United States.

In Baltimore, the population dropped 6,738 to 614,664.

Births outnumbered deaths in Baltimore, 8,654 to 6,871, and 3,402 people moved into Baltimore from foreign countries. But 11,008 people moved out of Baltimore to other places in the United States.

On the other side of the ledger, Maricopa County, Ariz., where Phoenix is the county seat, had the largest population gain of any county in the country.

“Maricopa County, Ariz., replaced Harris County, Texas, as the county with the nation's highest annual population growth, according to U.S. Census Bureau population estimates released today,” said a release by the bureau. “Harris County was the largest numeric gainer for eight years in a row.”

Census Bureau demographer Peter Borsella explained Maricopa’s ascendance to the top of the list for population gain, citing high levels of net domestic migration and “natural increase”—n.b. births outnumbering deaths.

"In the early 2000s, Maricopa County was in the top one or two counties by numeric growth,” Borsella said in a Census Bureau release. “From 2009-2011, Maricopa County saw much lower net migration than in the years before or after, which caused the county to drop out of the top population-gaining counties.”

"While net international migration has not reached prior levels, net domestic migration and natural increase have continued to rise, making Maricopa County this year’s largest numeric gainer,” he said.

From July 1, 2015 to July 1, 2016, Maricopa County’s population climbed by a net of 81,360 to 4,242,997.

Like Cook County, Wayne County and Baltimore, births outnumbered deaths in Maricopa County, and people moved into the county from foreign countries. But Maricopa also had an influx of population from other parts of the United States.

Births outnumbered deaths in Maricopa County by 56,073 and 10,188 people moved to the country from foreign lands. But the county also had 43,189 people move there from elsewhere in the United States.

Harris County, Texas, where Houston is the county seat; and Clark County, Nev., where Las Vegas is the county seat; ranked second and third for largest population growth from 2015 to 2016.

Harris County saw a net increase of 56,587 people to a total population of 4,589,928—this was even though the county saw a net of 16,225 residents move to other parts of the United States.

The population growth in Harris County was driven by a combination of domestic births and foreign migration.

Births outnumbered deaths in the county 73,072 to 26,660 and the county had an influx of 27,922 people from foreign countries,

In Clark County, births outnumbered deaths 27,352 to 16,501. At the same time, 27,735 people moved into the county from elsewhere in the United States and 6,566 moved into the county from foreign countries.

When measured by which was the “fastest growing” by percentage of population increase as opposed to “largest-gaining” in actual population numbers, The Villages metropolitan area in Florida led the nation with 4.3 percent population growth from July 1, 2015 to July 1, 2016.

The Villages, according to the Census Bureau is an area to the west of Orlando.

The second fastest-growing metropolitan area in the country during the year was the Myrtle Beach-Conway-North Myrtle Beach area of South Carolina and North Carolina. It grew by 3.9 percent.

The third fastest-growing metropolitan area—at 3.6 percent—was the Bend-Redmond area of Oregon.