Some foods that are toxic to chickens are onions, avocados and chocolate. Nightshade plants such as potatoes, tomatoes and eggplants have a toxic substance that is harmful to chickens.
Avocados contain a toxin called persin that is harmful to chickens. It causes the chickens to develop heart problems and difficulties in breathing leading to death in a short time. The toxin resides in high concentration in the avocado skin and the seed; the flesh has lower levels while the leaves are the most toxic.
Chocolate contains methylxanthines theobromine and caffeine. These toxic substances are also found in coffee, tea and colas. They cause various heart problems ranging from an irregular heartbeat to full cardiac arrest and eventual death within 24 hours of eating the chocolate.
A toxic substance called thiosulphate found in onions destroys the red blood cells and in excessive amounts causes jaundice, anemia or even death. Garlic, however, has health benefits even though it is in the same family as onions.
Plants from the nightshade family such as potatoes and tomatoes have a toxic substance called solanine that is harmful to chickens. Raw potatoes are hard for chickens to digest while the green parts of potatoes and sprouts are toxic. Solanine destroys red blood cells and can cause diarrhea and heart failure.