Insect?

I've seen this thing flitting around our four o' clocks in years past. This isn't the best picture I've ever gotten of it, but it's quite interesting to watch. Since four o'clocks bloom nocturnally, they provide a good opportunity to catch this guy in action. It looks like an insect from the back, but it flies around much like a hummingbird. Has anyone else ever seen one? Can you tell me what it's called?