Pet cockatiels, like children, don't always know what's best for them. They can become "seedaholics" and refuse to eat anything else; but unless you are feeding a pelleted food that contains fruits and vegetables, they need fresh food daily to reproduce the naturally varied diet they would eat in the wild. Keep providing this every day -- it may take a year or more to convert your birds to a better diet.
Cockatiels enjoy all the staple year-round supermarket fruits such as bananas, apples and grapes. Seasonal fruits such as nectarines, peaches, apricots, pears and strawberries bring welcome variety. Tropicals such as papayas, mangos, guavas and kiwi fruit are a good choice. Melons such as cantaloupe and watermelon are well received. Cherries may turn their droppings dark red -- it looks like blood, but is harmless. Feed less fruit overall than vegetables, as it is mostly sugar and water, although citrus fruits such as oranges and grapefruit do provide vitamin C. Wash all fresh fruit, cut into small pieces, and remove all cores, stones, large seeds and pits. If you have no fresh fruit, try dried fruit such as prunes, cranberries and raisins, as long as they are unsulphured.
Offer ample greens such as romaine, leaf lettuce, bok choy, kale, Swiss chard, watercress and mustard greens. The darker the colour, the more nutrition they contain -- iceberg lettuce is mostly water. Yellow and orange vegetables such as sweet potato and winter squash provide vitamin A, but your bird may prefer these cooked to raw. Carrots should be chopped or grated. Other vegetables such as runner beans, peas, corn on the cob, zucchini, tomatoes and Brussels sprouts can provide variety. Broccoli, spinach and parsley are frowned on by some experts because they contain substances that interfere with calcium absorption, so limit these. Even hot chiles are fine because birds don't taste the heat, but you may if your birds kiss you. Wash all fresh vegetables thoroughly and cut into manageable bits. Thaw frozen vegetables completely. Cool cooked vegetables before serving them. Canned vegetables provide little or no nutritional value at all, other than calories.
Alcohol, avocado and chocolate can kill cockatiels. Other foods on the forbidden list are eggplant, cabbage, caffeine (tea and coffee), milk and cream, raw potato, and rhubarb (all parts, including the leaves). Do not give any pet, or allow them access to, any part of a houseplant.
If you are not certain if a fruit or vegetable is good for cockatiels, check with a bird expert or a veterinarian experienced in bird care before you offer it. If you cannot verify that it is OK, don't include it.