Busiest birds around!
Sorry about the soft-focus feet.

Very nice shot. Are the feet moving or is the range of focus really that narrow? Care to share details?

Member of the Paridae Family. All these birds are very vocal and active. Very smart group of birds. These guys say "peter,peter,peter". They have complex alarm signals that have been shown to be known by other bird species nearby. When they say big predator overhead a differing response occurs than when they say a small one is overhead.
My favourite in the Paridae Family is the Black Capped Chickadee.

Sorry thought I had replied:
Shot at f/7.1 400 mm @ 1/125.

I think it must be a DOF issue the front foot is certainly in better focus than the rear.

Not sure tho

I agree. If you look at the branch the point of focus comes forward of the bird quite a bit. I think the back of the the DOF range is just at that back foot.Because the foot and the branch share that slight blur.
You know, things like this are more obvious to you than to anyone else. I know that I see things in my photos that bug the heck out of me and other people say "What?".

Durwood:

I like this shot. The ever so slight soft focus on the bird's right foot is not at all distracting for this non-birding type.

You must have been really close (10-20 ft?) to end up with such a shallow DOF at f/7.1 .