Female Hooded Mergansers have yellowish bills which are slightly darker on the upper surface and towards the tip. They also have brown eyes.
It was pointed out to me by sharp-eyed Ranger Kristin that one of our “female” Hooded Mergansers has an all dark bill. The bird in question also has bright yellow eyes.
The dark bill and bright eyes of the bird in the bottom photo make it an immature male merganser, not a female.
Good spot, Ranger Kristin!