Should We Be Worried

I've noticed that I can drive within a few feet of these sandhill cranes in the car and they don't blink an eyelid (if they have any to blink that is). But, when I walk within a hundred yards or so, they begin to get agitated. Must be something to do with predator instinct. 

I'm not going to get dressed in camouflage just to get a photo, I'll have to make do with observing at a respectable distance and back off before they are spooked. If they begin to call out, I know I've gone to far, but I try to stop before then.