Please forgive my ignorance - I'm a brand new birder & have only been observing for a little over a month.
I saw these ducks yesterday (2/6/18) in the pond in Prospect Park, Wheat Ridge. My best guess is that they are Mallard-Northern Shoveler hybrids. They resemble male Mallards, in that they have green heads, yellow bills, and those curly "butt" feathers (I'm sure there's a more proper term for those). But they were significantly larger than a Mallard they paddled by - they were about the size of a cackling goose that was also nearby. And then there are those white chest markings. Northern Shovelers have white chests, and the front duck's bill definitely looks Shoveler-like in shape. But Northern Shovelers are even smaller than Mallards. Could the offspring of a Mallard-N. Shoveler mating be larger than either species? Do these species even "commingle"?