Spark has an obsession with chewing on/eating sticks, bark and rocks. When were at the ballpark he always manages to find a piece of bark or a little rock. I think he chews on the rocks because they help to cut-in all those sharp puppy teeth. The bark and sticks are maybe to add more fiber to his diet?
KSDS PIT Hosta also really enjoys bark and sticks- he loved it when he visited us because we have LOTS of sticks and bark in our backyard. Isn't that strange another thing Spark and Hosta have in common. They are so alike its freaky!
Here are some pictures of my little 'fox' pup enjoying sticks and stones in the backyard: