eBird, I was perplexed with the choice of checking American Coot or American Coot (Red-shielded). My research into this conundrum indicates that almost all North American coots have a red callous, or shield, on the top of their bills. This coot, found in Northfield on 13 October 2016, shows this field mark.
Coots found in the Caribbean have white frontal shields. Ornithologists do not know if these Caribbean Coots are a separate species or if they are just a color morph of American Coots. Caribbean birds occasionally show up in Florida. Payne and Master (1983) found a successfully breeding mixed pair of coots in Michigan. They could not confirm that the white-shielded bird was a Caribbean Coot, but concluded that their ability to interbreed suggested that red-shielded and white-shielded birds are, indeed, but color morphs of a single species.
Why the choice in a Minnesota eBird checklist? Often the shield color is hard too see in poor light or from even moderate distances. A white-sheilded coot is quite unlikely in Minnesota, or anywhere north of Florida. But they are not impossible.