I have 3 barn cats who have never once touched my chickens. Even when I had chicks in the barn with their mom. One of my roosters is currently living in the front of my barn until I can build him his own coop, and my cats haven't laid a paw on him. I even have a hen who is half blind in both eyes who hangs out in the front of my barn where the cats live. They all get along great! I think as long as the cat has plenty of cat food, and isn't a feral cat, your chickens should be fine. Even when I had 9 cats last year, they never bothered my chickens. My chickens all free range too. I wouldn't worry too much, as long as the cat isn't feral, and your chickens are at least 2 months old. That way your cat will be able to tell it's a chicken, not a bird.