Bruce,
This website suggests that they migrate:
http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1144&context=icwdm_usdanwrcIt also says:
"Most of the interior populations are located from the Great Lakes west across the northern prairies of the United States in Canada."
Considering that you're in the "Upper Central Midwestern Great Plains," that puts you in their neck of the woods.
P.S.- Don't tell the cormorants on the Gulf Coast that they're the minority. They sure don't breed/act like it.