"Intersex" birds are those that show plumage and bare-parts colour seemingly intermediate between male and female. This is commonly seen in Mallards and is also known to occur in other ducks and perhaps other birds. Such birds are often confused with hybrids. As far as I know, the phenomenon is not well-understood or studied.

I use "intersex" in quotes because the term has quite a different meaning to how it's used in humans.

Here's an excellent collection of images: https://birdhybrids.blogspot.ca/2015/01/intersex-birds-and-their-confusion-with.html

This project is for any ducks thought to be or to possibly be "intersex". I'd like to get a better handle on how their plumage can vary.

Other interesting questions:

Can these birds reproduce, and if so which sex are they? (Generally thought to be female, but I'm not sure how well-supported that is).

Is behaviour and voice typically more male-like or female-like?

Does the plumage remain consistent over the birds life? (The answer is at least sometimes yes.) less ↑