Chickens can only eat about a cupful of food a day. That cupful therefore has to be pretty high in nutrition. Some scraps (like lettuce) just aren't very nutritious.
Potato peels aren't well digested and don't count for much, but if you boil them then you've instantly made them chicken friendly.
Other useful scraps are stuff like kale/spinach, leftover omelet, curdled milk, meat and so forth. Mashed potato leftovers are a good treat as are small amounts of cheese (very small because cheese is salty). They quite like porridge and if you add mincemeat to increase the protein, it's not a bad once-a-week food to encourage egg laying and give them a break from routine. My chickens have lately discovered fresh whole pumpkin, which is a bit of a drag as I was trying to grow them for the household, but never mind... High in vitamin A and good for yolk colour.
(Pumpkin peel is a little hard for chickens to eat, so like potato peel it's probably best cooked.)
Chickens can't tolerate too much salt, so beware of too much of anything like ham, bacon, salty chips, etc.
As a general rule people say it's best to feed a balanced ration and only scrap feed in small quantities. However I reckon it all boils down to how good your scraps are. Here we have pretty high quality scraps because I'm obsessed with feeding my children healthy food (to make up for whatever terrible thing happened to their digestive/neurological systems when they were infants), and we do have a lot of leftovers. Far better the chickens get them than they go in the bin.
Great topic, hope I haven't ranted.
