Cayuga may be sick???

[Pinx]

Not sure if I'm posting this in the right place. Forgive me if I'm not.

I have a Black Cayuga duck that is about a year old. She has looked perfectly healthy until 2 days ago. She looks very oily and while the other ducks have perfect looking feathers, hers look oily and look more stringy. I'm not sure if this is a sign of some sort of sickness, but she just doesn't look right. She is eating good, still laying an egg a day, but doesn't spend as much time in the water as the other ducks. They are in the water all day it feels like but she would rather walk around the edge and dig in the mud. The other ducks are only about 3 months old, so I don't think they are picking on her.

They have their little bit of water that they can all get in at the same time, then in the evenings they get to go out and play in the horse tank we buried for them. They play in that until they all get out and head to the coop. Still though, while the younger ducks stay in as long as possible she gets out and digs around in the dirt. She does get in the water for a little while but she doesn't stay long.

So does a dirty, oily, unkept duck mean she is sick?