Derek, If I had to take a guess. I would say that they are male Mallard-Black Duck hybrids. If you have access to a copy of National Geographic's "Field Guide to the Birds of North America" you can see an artist's rendition of this hybrid. I have had the opportunity to see and photograph a few of these hybrids up close usually with groups of mallards on Lake Superior. The streaking patterns on the head vary dramatically by individual leading to some pretty unusaul looking ducks.
Michael Cary Superior North Shore Images www.superiornorthshoreimages.com