Who's egg is this?

[JRMMC13]

I have a small flock with mostly production reds, but there are a few 'mutts' in with them. I have no clue what they are, other than one is at least part frizzle.
The egg in question is grey(ish) with some darker spots. It's next to the reds' eggs. Don't mind the messy pen, they've all been enjoying the 'pool' today.

I also have these ducks in the same pen.

 

[JRMMC13]

I didn't include a picture of the frizzle.