I only sit this still when I'm either really focused or really bored. This cat has, I think, discovered a new variety of invisible bird. It's important that he tracks it down to (eventually) pounce on it, or this new dangerous species could plan a hostile takeover, its invisible bird brethren flapping their wings all around us, crashing into our cars and planes, taking down our civilization!
Or, he's just a dumb cat and he's looking at a dumb thing. Or invisible mutant monster bird! One of the two.