I've come to the conclusion that while hummingbirds aren't particularly afraid of anything, they don't like the situation to change while they're at the feeder.

I can be working in the yard on a weekend like this, a hummingbird will fly right up to me, look me over, and then go to the feeder. But if I walk away it will fly away. If I'm not there when it arrives and then walk up, it will fly away.

But as long as nothing changes after it's cased the joint, it's fine with pretty much anything. Including a cat sitting four feet away and not even trying to keep still.