There are an embarrasing number of groups of birds that I truly am ignorant about when it comes to identification. Ducks are one of these along with sparrows and small shorebirds. To further compound the duck problem I find that mallards are considered the founding species of many other types of duck and they can still inter-breed creating variants. So for me - they are all ducks. These two (I'm saying mallards) seem to have joined the fitness craze as they were doing a number of stretches and exercises. If you would like to help with the identification please do so, I'm not saying I'll remember it the next time I see them but I will appreciate the effort on your part.