I never could get a clean shot of this bird.

You can see the bill is greenish-yellow, but with a dusky ridge. The green of the head doesn't reach the bill, and especially, just over the top of the back of the Gadwall sleeping just below it, is the white on the wing (inner secondaries, like Gadwall) that this bird showed. I saw this well once when the bird briefly flapped, before I had my camera out.