The bureau added that the 33,700-person drop was a relatively small percentage change compared to Illinois' population of 12.8 million people. The population exodus is likely much greater as the bureau's estimate doesn't account for the thousands of Puerto Ricans who left the island in the aftermath of Hurricane Maria in September.
Wyoming saw the largest percentage decline.
Most of Illinois' population decline was due to a net loss of 114,779 people moving to other states.
The distribution of seats in the House is based on the census taken every 10 years, with the next one scheduled for 2020.
Texas added almost 400,000 residents to its population during the past year, the most of any state.
Looking at the numbers in a different way, Texas leads in the numeric increase of residents, adding 399,734 people to its population over one year, with a total population of 28.3 million.
If trends continue, the state could find itself in a downward spiral, Harger says, though he doesn't believe IL has reached that point yet.
A state official says from 2016 to 2017, Wyoming saw its largest population decrease since 1989.
Still, residents moving to the US from other countries continue to be a significant factor in USA population growth as 1.1 million people moved to the USA over the previous year, according to the census bureau.
ME grew 4 percent, Rhode Island by 2 percent, and CT and Vermont basically broke even. That's an increase of more than 3 million from 2010. That and a net gain of 6,836 worldwide migrants made up for a net loss of 2,724 people moving to other states, according to the estimates. That would be Idaho, which is now home to 1.7 million people and grew 2.2 percent during the past year.