Aythya sp.

Pair of scaups
These seem to be a pair of scaups, but I have not the foggiest idea whether lesser scaup, Aythya affinis, or greater scaup, A. marila, Wonderland Lake, Boulder, Colorado, April 6, 2019.

To my eye the markings are the same, right down to the black mark on the nose. Hard to tell their size, but probably smaller than mallards. Boulder is on the edge of both their ranges. I am curious why 2 species would be so nearly identical – is there significant interbreeding, or did they simply bifurcate very recently?