My unfortunate experience with roosters defending the flock has been that (apparently, because I have never witnessed it so I'm guessing) the rooster gets taken first, and then the predator comes back and finishes its meal with hens. A rooster can be a help, but should not be considered the only line of defense. For those who don't wish to free range, but have a large fenced area, I would suggest also putting up electric fence. Put something over the top of a small run to keep aerial predators at bay. For free ranging, make sure they have plenty of cover.