If chickens eat your grass, you will end up having to reseed your lawn in those areas. This is when you discover that the chickens will also devour the grass seed that you plant before it can ever grow. There are certain things you can do to keep the chickens away from your grass seed, but you might have to try various measures to find the right one for you.
Place a thin layer of topsoil over the grass seed, just enough to cover the seeds. Use a garden rake to spread the seeds into the soil.
Put chicken wire over the seeded area and stake it down so the chickens cannot get near the seed.
Lay about an inch of mulch over the seed if the topsoil does not work. Mulch will deter the chickens.
Consider installing chicken wire fencing around the newly seeded area to keep the chickens away. Divide the area into multiple sections if necessary.
Offer the chickens some of their favourite foods so they do not want to eat the grass seed. Make sure that they do not run low on food, which can cause them to wander into your seeded area.