The skins of raw potatoes contain substances called alkaloids that can be toxic in high amounts. There are more of these substances in the greenish-colored skins than in the normal brown, yellow or red skins. Cooking mostly destroys these substances.
All raw potatoes contain the alkaloids in varying amounts, to differing degrees...even among potatoes of the same type. The greenish skins are found on potatoes that have grown at the surface of the soil, instead of beneath the soil.
It's the dose that makes the poison. The dose needed to cause problems in a 6 lb. chicken is lower than the dose needed to cause problems in a 170 lb. human. So, the occasional raw potato that is high in alkaloids might cause problems in my flock. Since I can't look at a potato and tell if there are lots of alkaloids, I play it safe by providing only cooked skins to my birds.