There are a few YouTube videos of people (Karl Hammer?) feeding only restaurant waste food to their chickens. Since they get so much waste food, they don't bother to worry about the small amount of food that might not be good for the chickens to eat. Evidently, their theory is that if there is enough food to chose from, the chickens will naturally stay away from the food waste that is not good for them. In that case, those food waste products not eaten get composted, eaten by worms, and then the chickens can eat the worms.
Yes, the abundance is a key. If they have lots of choices, they do a good job of picking and choosing what they want/need and avoiding what they shouldn't eat. If all they have are bad options, they will potentially eat them.
I'm looking forward to getting back to food pickups...and I think the flock is too...they love it!