You have flyers then. You can get them at your local feed store like Theisen's which is where I get mine: https://www.theisens.com/products/purina-flock-block25-lb
This is probably a better brand:
https://www.kalmbachfeeds.com/collections/blocks-tubs/products/all-natural-poultry-block
I've...
I would get a roll of wire or plastic mesh to run around the inside of your existing fence if you can afford to and the parents don't mind. It's much easier for them to demolish a smaller area than a large yard. But for now maybe you can pick up one of those flock blocks?