Now that I go back through This thread I see a lot of serious stories too so I'll add mine.
I once saved one of my cats from a dog that had his head in a death grip. I literally pried that dog's mouth open. This was a medium sized dog too, under 50 lbs, NOT big at all. My cat survived with several teeth that were broken from being CRUSHED and a broken jaw. My hands were so wounded from being bitten (my hands vs. a dog's jaw that is clamping down, that was brutal) that I could barely even turn a doorknob for over a week. Driving, writing, opening, or picking up ANYTHING was painful. I had holes in the center of my fingernails from where the dog's teeth were clamping down that took 4 months to fully grow out. I have prominent scars all over my left hand from this.
I love my animals, as their caretaker it is my job to protect them, I would do it again in a heartbeat if any of my animals were under attack. I just want to express the power behind a dog's jaws. I mention it because I was able to intervene immediately in this situation and with a $1,200 trip to the vet my cat luckily survived.
However, if it was a chick it would all be over instantaneously. Even if it was a chicken and you could intervene (maybe with a very small dog?) the injuries would be severe if not fatal, then you have a suffering chicken and a painful choice to make. Once you witness or experience your animal being severely injured or killed by another animal you never leave it up to chance again. I have to live with the guilt of my cat's injury and I blame myself for thinking that a dog would never do anything like that. I have dogs and I love my dogs deeply but I am much more cautious now. I am very protective of my chickens. When your animal is injured or killed because you "trusted" you have no one but yourself to blame.
Just be cautious, be careful, and never leave your birds in the company of a larger animal unless you are closely supervising.