WHAT TO BRING
Dress is informal in this rural state. Levis are the rule rather than the exception. When birding in spring and fall, carry a warm jacket because the temperature can drop in a hurry. As for winter dress, there is not much to say other than “get the warmest clothes and foot-gear possible.” It is next to impossible to overdress for Dakota winters. Other things that could come in handy in winter are: 1) a shovel (especially if you intend to drive the back roads), 2) studded tires, 3) booster cables, and 4) a lighter-weight motor oil for better cold-weather starts. Make sure to winterize your radiator to at least -30°, and keep more anti-freeze on hand; you may need it.
As a final suggestion, if you own a spotting scope by all means bring it. There are certainly few activities more frustrating than trying to identify shorebirds on a distant mud-flat with nothing more than binoculars. A scope can also come in handy for viewing some of the more elusive prairie sparrows.