It's a great shot, but humans should NEVER feed squirrels like that. It's very dangerous and doesn't do the animal any good. And it's illegal in many places (like parks).
"Why 1/6400?" probably on aperture priority, shot wide open and that's what the meter called for. The question I would ask would be why ISO 800, but shooting close range with a 600 requires a much higher shutter speed than you might think, and this pretty much guarantees no motion blur, or at least not much.
Thank you so much for all your comments, nobody shooting people with 600mm , the setting f/4.0 1/6400s 800 ISO -0.3 EV for shooting Humming birds not for She, just happen I see her feeding so cute and I don't have time to change the setting
