You can start feeding your Cockatiel pretty much any fresh Veggies and dark, leafy Greens EXCEPT for any Veggies in the Onion family, as they are all toxic/poisonous to birds. So no Onions of any kind, no Chives, no Leeks, etc. And you shouldn't give him any of the "watery" lettuces, such as Iceberg Lettuce or any of the Cabbages that are very watery or light in color, simply because all they provide to your bird is water, that's it, they contain very little to no nutritional-value at all...Instead, you want to stick to all of the dark-colored, leafy Greens such as Mustard Greens, Collard Greens, Turnip Greens, Dandelion Greens, Arugula/Rocket, Chards, Escarole, Endive, Bok Choy, Pak Choy, Broccoli Rab, etc. These are all full of all kinds of vitamins, minerals, and even some protein. As far as fresh Veggies go, again, pretty much anything except for anything in the Onion family or Leeks, Chives, Garlic, etc. Some favorites include Green Beans of all varieties, Pea Pods of all varieties, Bell Peppers of all varieties/colors (Green, Red, Orange, Yellow, etc.), Broccoli, Brussel Sprouts, all kinds of Squash including Butternut, Acorn, Spaghetti, Yellow, and Green Squash (Zucchini), Okra, Eggplant, different varieties of Carrots, etc. Also, birds do not possess the receptors on their tongues/taste buds for capsaicin, so they cannot sense any of the "heat" from any types of peppers. They love the Bell Peppers and the Bell Pepper seeds for the flavor, and they aren't at all "hot", but they also love all different types of Peppers and their seeds, even the very, very hot Peppers that we have trouble eating because they cannot sense that they're hot. So you can try whatever different varieties of Peppers and their seeds that you have access to without worrying about them being "too hot" for your bird.
***Only feed your Cockatiel FRESH Veggies and dark, leafy Greens (and Fruit), no canned Veggies or Fruit at all, as they have lost most if not all of their vitamins and minerals, and they have a ton of added salt (Veggies) and sugars/syrups (Fruit)...And frozen Veggies and Fruit aren't optimal either, so try to stick to only feeding FRESH Veggies, Greens, and Fruit.
As far as fresh Fruit goes, it should not be fed every day, and it is not interchangeable with fresh Veggies and dark, leafy Greens, which SHOULD BE FED EVERY DAY. A lot of people mistakenly think that fresh Fruit is the same as fresh Veggies and Greens are, and that they are equally as healthy and good for their birds, and that isn't at all true, because all Fruit is absolutely full of all kinds of natural Sugar, which is turned into fat and stored as fat in their bodies, specifically in their Livers, just like the fat from the Sunflower Seeds, Nuts, and Dried Corn is. So fresh Fruit should only be fed to your bird 2-3 times a week and in small-portions, whereas fresh Veggies and dark, leafy Greens should be fed to your bird every single day.
***You can pretty much feed your bird any type of fresh Fruit except for Avocado, which is toxic/poisonous to birds, just like Onions are. Also, you should try to stay away from all types of Citrus Fruit, such as Oranges, Tangerines, Nectarines, Grapefruit, Limes, Lemons, etc., because all Citrus Fruit is extremely high in Vitamin C, and birds should not be fed much Vitamin C at all, because Vitamin C enhances the body's absorption of Iron naturally and automatically, and birds do not need nor can their bodies handle absorbing much Iron at all...If a bird absorbs much Iron at all, they will become extremely sick with a disease called "Iron Storage Syndrome", which is typically fatal without immediate medical treatment from an Avian Vet.[//B] They go into a kind of toxic-shock from the high level of Iron in their bloodstream, and they will die pretty quickly without medical treatment, and often medical treatment doesn't help. So for this reason you need to stay away from feeding your Cockatiel much, if any Citrus Fruit at all, nor giving them any types of Vitamin Supplements that contain Vitamin C or Iron.
However, there are plenty of Fruits that are completely safe to feed your bird as an occasional treat a few times a week without feeding them any Citrus Fruit...Favorite fresh Fruit include all types of Berries (Strawberries, Raspberries, Blackberries, Blueberries, etc.), Cherries (NO PITS!), Bananas, Plantains, all varieties of Pears, all varieties of Apples, all varieties of Grapes (NO SEEDS!), Apricots and Peaches (NO PITS!), and then all of the "Tropical" Fruit such as Mango, Papaya, Guava, Star Fruit, Passion Fruit, Kiwi (NO SEEDS!), Coconut (IN SMALL AMOUNTS!), etc.
***[B} As opposed to feeding your birds the seeds from Peppers, which are good for them and safe, NEVER GIVE ANY SEEDS OR PITS FROM ANY TYPE OF FRUIT AT ALL TO YOUR BIRD! MOST OF THEM ARE VERY POISONOUS! So just stay away from ALL seeds and pits that come from any and all types of Fruit and you'll be safe. Only give your bird the flesh of the fresh fruit.
***As far as your Cockatiel's daily staple-diet, which apparently now is a low-fat seed-mix, as long as the seed-mix that you feed your Cockatiel contains NO Sunflower Seeds, NO Nuts of any kind, and NO Dried Corn at all, and as long as it does contain a wide array/variety of lower-fat seeds, grains, legumes, herbs, veggies, and a small amount of fruit, then it's every bit as healthy for your Cockatiel as any "Fruit-flavored" (multi-colored) or "Nut-flavored" Avian Pellet staple-diet is, because the Fruit-flavored (multi-colored) Avian Pellets contain so much added, artificial sugars in them that they end-up providing your bird with as much fat being stored in their bodies/livers as the cheap, junky seed-mixes that contain Nuts, Sunflower Seeds, and Dried Corn do; the same goes for any of the "Nut-flavored" Avian Pellets, which typically contain a ton of natural oils from actual Nuts, which are nothing but fat (Zupreem is one of the only brands that makes a "Nut-flavored" Avian Pellet, but many make the multi-colored, "Fruit-flavored" Avian Pellets, and all of them are terrible for them).
Now if the seed-mix that you're feeding your Cockatiel contains any Nuts (specifically Peanuts, you should not ever feed your bird ANY Peanuts, as not only are they very high in fat and provide little nutrition, they also carry a lot of lethal strains of mold and fungi), or if it contains any Dried Corn at all, then it's not a healthy seed-mix, and you need to either switch it to a low-fat, high-protein, varied seed-mix that does not contain the Sunflower Seeds, Nuts, or Dried Corn, as this will be much better for your Cockatiel than any Fruit-flavored Avian Pellet, OR you need to gradually transition your bird from the seed-mix over onto a "NATURAL-flavored" Avian Pellet, such as Harrison's, TOPS, Zupreem Natural, LaFaber Pelletberries, Tropican Granules, etc. If you were thinking of switching your Cockatiel from a low-fat seed-mix to a Fruit-flavored Pellet, then you're actually going in the wrong direction, because a low-fat, high-protein seed-mix as their staple-diet combined with at least one serving of fresh Veggies and dark, leafy Greens each day is much healthier than any of the Fruit-flavored Pellets are, due to their extremely high sugar-content.
