
What he sees he keeps to himself. Boz rarely barks. I've heard his formidable bark about five times in the seven years he's lived with us, and at those times I give his declarations great respect: "What is it, Boz? Good boy, Boz!" Usually it's nothing--a passing pug, an insistent crow, a breeze.
I could be wrong about this, though. He might be joining the Twilight Bark.
(The video I've linked you to is 10 minutes long. The Twilight Bark begins about three minutes in. The whole thing is worth watching.)