Right behind Idaho with increases were Nevada at 2 percent; Utah, 1.9 percent; Washington, 1.7 percent, and at 1.6 percent each, Florida and Arizona.
As for why the decline is happening, Harger said it could be linked to a declining birth rate for the state, a drop in domestic migration - meaning people coming to IL from other states - and a "leveling off" of foreign migration.
NY state had a slight population increase, adding 13,113 new residents, which is about how many people live in Schodack, to reach 19,849,399, according to U.S. Census Bureau's latest state and national population estimates released Wednesday.
Census officials confirmed IL lost its spot as the fifth-largest state in the wake of Pennsylvania's modest population growth.
Behind Wyoming's decline rate, which was the largest in the nation, were West Virginia, Illinois, Alaska and Hawaii. Florida comes in at number three, pushing NY down a peg to four when compared to counts from the 2010 Census.
The Census Bureau released on Wednesday the latest population estimates by state. Persons younger than 18 now constitute 22.7% of the state's total population, compared to 23.6% in 2010 and 24.6% in 2000.
A state official says from 2016 to 2017, Wyoming saw its largest population decrease since 1989.
Migration from other states and countries accounts for the majority of the population increase, according to the data. The previous year, Idaho grew by 1.4 percent. California is by far the most populated with 39.5 million residents. For domestic numbers, MA saw a loss of 23,000 residents moving to other states. Texas follows in second with 28.3 million people.