This came from another page on this site got some good information on what not to feed your chickens.
Don’t feed the following things to your chickens:
(I'm sure people have experienced exceptions to this list, but if we want to raise our birds the best way possible, "better safe than sorry".)
Raw green potato peels
Toxic substance called Solanine.
Anything real saltyCan cause salt poisoning in small bodies such as chickens.Citrus
Dried or undercooked BeansRaw, or dry beans, contain a poison called hemaglutin which is toxic to birds.Avocado Skin and Pit
Skin and pit have low levels of toxicity.
Raw eggsYou don’t want to introduce your chickens to the tastiness of eggs which may be waiting to be collected in the nestboxes.Candy, Chocolate, SugarTheir teeth will rot… No, it’s just bad for their systems, and chocolate can be poisonous to most pets.
A quote from Nifty-Chicken, the Administrator of BYC:
"I gave up on my birds knowing what was best for them when I caught them all eating a block of Styrofoam pellets."
Regarding toxicity, the following is copied from a post by DLhunicorn on May 14, 2007 in a thread titled "Potato Peels". (Thank you DLhunicorn for your tremendously helpful and knowledgeable contributions to BYC!)
"Do not count on your chickens "knowing" what is bad for them...also do not count on these "toxic" plants immediately being identifiable by finding a dead bird the next morning...usually it is a slow process damaging organs , inhibiting the ability of your bird to utilize the nutrients in their feed, etc.
Herbs and spices are generally safe for your chickens, and despite having a very limited number of taste buds, they’ll pick around what they don’t like.
Be aware that not all fruits and vegetables are safe for chickens to eat. Below is a list to help make sure you keep your birds safe.
Greens are great for hens, as they can result in darker, richer yolks. Lettuce, kale, turnip greens and chard are great greens options. Watermelon, strawberries and blueberries are common fruits.
More importantly, there are foods to avoid feeding your birds:
• Onions and garlic are not recommended, as they can give your eggs an off-flavor
• Avocado pits and skins contain a potential toxin called persin
• White potato peels that have green areas contain a toxin known as solanine, which can be fatal
• Undercooked or dried beans contain a toxin known as hemagglutinin
• Rhubarb contains anthraquinones, which can have a laxative effect. Rhubarb damaged by severe cold can be poisonous to chickens, due to the high concentration of oxalic acid
• Never feed moldy or rotten foods to your birds
• Very salty foods will result in excessively wet feces and may be toxic if enough is eaten
And remember to follow the 90/10 rule: 90% of the diet should be a complete feed and 10% can be treats and snacks.