Another way to make a barking sound is just two small hard stones struck against each other. Nice rounded stones from a stream work great.
With practice you might be able to do a softer call by making a high pitched cluck sound in the back of your mouth. I can do it but my brother and father can't.

And don't forget a pocket full of stones for tossing. Get a squirrel up a lone tree and it keeps on the opposite side as you sneak around to get a shot, sit down a while then skip a stone into the leaves or brush on the other side and you might trick him into moving to where you can shoot him.