My experience:
5 chickens in a 2,000 sq. foot area that is just weeds (where the weeds completely die in the summer and sprout in the spring): My girls kept all the weeds completely down because they would eat the sprouting new weeds while they were young and would scratch around leaves that acted as mulch and made it hard for stuff to grow.
My brother:
5 chickens in a backyard with about 1,000 sq. ft of well established grass: No effect on the grass. Keep in mind there were plenty of other places in their yard (some dirt / weeds) for them to scratch around in. He also had a garden that he would section off when the plants were young but he said they were fine once established to the point he could remove the section.
5 chickens in a 2,000 sq. foot area that is just weeds (where the weeds completely die in the summer and sprout in the spring): My girls kept all the weeds completely down because they would eat the sprouting new weeds while they were young and would scratch around leaves that acted as mulch and made it hard for stuff to grow.
My brother:
5 chickens in a backyard with about 1,000 sq. ft of well established grass: No effect on the grass. Keep in mind there were plenty of other places in their yard (some dirt / weeds) for them to scratch around in. He also had a garden that he would section off when the plants were young but he said they were fine once established to the point he could remove the section.