My cats watch intently, and playfully bat at the chicks with their claws still safely tucked away, then go to sleep close to the brooder because it's warm.
However they kill wild birds occasionally and I don't trust them around the babies. My chickies will be ranging in a tractor that is hopefully quite secure.
I used to have outdoor only cats who killed birds, rabbits, squirrels, moles, small snakes, you name it - so feral or outdoor cats I would be even more afraid of for small chicks.