Chickens have been feeding themselves for thousands of years without supplemental feeding, even domestic chickens. Obviously if the ground is frozen or is covered with snow, it is a different situation, but they don't absolutely have to be fed to do very well.
It also depends on the quality of forage. If all they have to eat is off a manicured lawn where the grass never goes to seed, they don't get a real good balanced diet. But if they have access to different grasses and weeds, grass and weed seeds, and various creepy crawlies they scratch up or catch, they can find themselves a really well balanced healthy diet, plus they are physically in good shape because they get exercise.
I find with mine they don't eat much of the prepared food when good forage is available. I don't know your exact situation, but I would not worry if it is available to them. They'll do OK.
