The squirrels have totally figured out the cats can't get them. I swear sometimes they intentionally taunt the cats!
Actually, we put a birdfeeder right at cat tree height. The squirrel climbed up the piece that separates the deck doors and kind of "flopped" over to the bird feeder. He's stretched out between the separator and the feeder here, but he pulled himself over and just sat in the feeder eating.