I thought this was one of those things that everybody knows. But the Dude didn't. So it may not be quite so well-rooted in the British subconscious as I had assumed...
In the picture above, there are three mallards, a pigeon, a coot and a moorhen.
How do you remember which of the little, black water birds is a moorhen and which one is a coot?
No-one is ever described as being as bald as a moorhen.
The coot is on the right.