Underneath our walnut tree are many, many, old walnut shells. Many are split in half and the edible nuts inside are long gone. This one was a bit unusual though- it had this hole on both sides of the shell.
Looking closer, the marks on either side of the hole look like grooves. I am guessing that these marks were made by squirrel teeth. The nut inside is gone, so whoever made these holes probably got a nice meal out of it.
A bit further into the woods behind our house, I watched a squirrel with a mouthful of leaves climb this Douglas Fir tree. It stopped at this branch and then disappeared into the clump of leaves and sticks that you can see here. A squirrel home, perhaps?