This differs a little if we're talking about turkeys, geese or other poultry, but the basics are the same: poultry like to eat growing grasses, like clover, buckwheat and Kentucky bluegrass. They eat broadleaved weeds of all kinds. They eat the growing tips as well as the seeds of these plants.
Chickens also eat earthworms, insects, and slugs of all kinds. Finally, they need to eat a little grit - sand and/or coarse dirt - to keep it in their gizzards to help them grind up the wild food they forage.