First you need to understand how a birds gizzard works. Basicly it is a super powerful muscle the contracts on itself to grind the food before it passes through the intestines for digestion. The gizzard does not digest or break down foods like the stomach does. Since a chicken does not have teeth to grind their food like we do, the gizzard does this job. Chickens seek out small stones about the size of a cherry pit and eat them where they enter the gizzard and help in the grinding of the food. Without this grit, it makes it much harder for the bird to grind scratch grains since these are generally whole grains,, not pre-ground like most pellets or mash that pretty much turns to mush with just a little added water that the birds drink. Most lawns do not have many of these small stones lying about,, so to answer your question,,, yes,, they should have grit. A bag of Cherrystone grit is fairly cheap and will last a very long time. Offer it in a small container near your normal feed as free choice.. The birds will eat as much as they need,, perhaps a lot at first if they have never had any but it will taper off as they get enough.