Chicken feed is not very expensive most places. I would recommend buying a bag of Layer Feed or All Flock at your local feed store or TSC and give them free access to it. They should have access to their feed 24/7.
I just picked up some birds that were not fed regularly and they ate like there was no tomorrow for 3 days in a row. I was worried they were eating too much, but after they got their fill, it slowed down a quite a bit. Birds need the right nutrients to grow healthy and lay eggs. If you choose to get the All Flock, then you need to offer oyster shell on the side.