What Am I Not Hearing?

Hey, so I take  my cat for walks. Not on a leash. I just leave the yard and head out into the long grass that surrounds it, and he takes off. He sticks pretty close, except for when we get too close to a tree, and then he promptly scampers up it, ears back, a hint of the maniacal to his head whipping left and right, eyes round, tail twitching.