With the bare eye first we thought a bird, then saw it had a tail, then decided it was a small mangled squirrel. Examining all our shots good and bad - it was a chipmunk.
The more interesting thing is that, although the zoo's earlier pictures show two ear tufts each, ours show that one tuft has disappeared. I presume it was just rubbed or bitten off and will grow back.
(best shot showing both kittens, the tufts, and the chipmunk was taken by my friend, posted with permission)