Cats are pretty good at looking out for themselves, but not much else. We have one that broke into the brooder and ate some chicks, but leaves the adult birds alone. The only protection she offers them is the distraction factor!
We have a cat, Fiesty, who we've found sleeping with the chickens! We go to open the coop in the morning, and there she'll be, in an unused nest. She's also a great hunter - once our dog, Blue, chased a rat and it ran through the hogwire fence, where Fiesty took up the hunt/chase - it was pretty cool to watch! But I leave the "guarding" to our GP, Ludwig.