When you try to pick them up. There have been several posts of late complaining of this behavior. To explain this I've made a 10 second video so that we may see from the bird's point-of-view. Oh no! The Big Monster is trying to grab me! Run Away! Run Away! As you can see, it's a defense mechanism. After many generations that characteristic has become a mandate. Those that didn't have it, didn't survive. Yes, it's possible to condition them not to react this way, but are you sure you want to? It is after all, a survival trait.