One thing to keep in mind in Michigan is the difference between State Parks/Recreation Land and State Game Areas.
All hunting in Michigan's State Parks/Recreation Lands is forbidden from March through September. Even for the unprotected species.
On the other hand, you can continue to hunt unprotected species in Michigan State Game Areas, with a small game license, year round.
To find these State areas in your county,
click here. Click on your county to open a PDF map of your area, then zoom in to see details (it's high resolution). Save it to your hard drive or PDA for easy access. I keep these maps on my smart-phone and combine them with GPS while I'm afield to keep me from wandering onto private property.
Green areas are open to hunting all year for unprotected species and extended seasons like fur-bearers and whatnot. As Ed said, Reds, Chucks, Porkis, Skunks, Thirteen-Lined Ground Squirrels and Possum are unprotected and open all year.
Of course, if you have private property to hunt, you don't need a license to hunt the unprotected species... and if you want to bend the rules, you can shoot squirrels and bunnies and such as pests but I don't condone that unless they really are ripping up your garden or whatever.
You may already know all of this but perhaps someone else will come along that doesn't, so here it is.