Have you ever caught your dog barking at nothing in the backyard?

My dogs bark at cars, people, and sometimes airplanes. But recently I have noticed that one of them just likes to bark at air. I have peaked around the corner a few times and I have noticed that he likes to look up in the air and bark. This generally sets the other four dogs off and then there is a crescendo of barking. I really hate it when the howling starts. Well, I always pull the dogs inside and put them in their day crates. They love it in their crates so, I guess, I reward them for being noisy. But, I don't know what else to do because yelling "Shut up," doesn't work--I learned that lesson years ago.