A varied diet of fresh vegetables, a smaller portion of fruit, and a quality pellet base generally serves most pet birds well, supplementing (not fully replacing) seed mixes, which tend to be high in fat if fed exclusively. Leafy greens, carrots, broccoli, and bell peppers are great vegetable staples, while fruits like apple (seeds removed), berries, and melon can be offered as occasional treats.
Avocado is toxic to birds and should never be offered, along with chocolate, caffeine, and alcohol, all of which affect birds even more severely than mammals due to their fast metabolism. Onions and garlic in large quantities can also cause problems over time.
Fruit pits and apple seeds contain trace cyanide compounds and should be removed before offering the flesh. As with any pet, introduce new foods gradually and remove uneaten fresh food within a few hours to prevent spoilage in the cage.