An open pond is ahead, just beyond that clump of willows. You slow down, trying to walk silently, as you approach the open water. Oops, you weren't quite silent enough. Two ducks jump from the pond and disappear through the trees making a long whistling call that rises in pitch as they fly. You had just time enough to see that they were not quite as big as a Mallard, and that they appeared to be pretty dark in color. What kind of duck were they?