A budgie's diet should consist of a quality low-fat seed mix, fresh vegetables (celery, silverbeet, broccoli, capsicum, corn, etc.), occasional fresh fruit like apples (although they are not big fruit eaters), millet sprays as treats, and constant access to clean water. Please don’t forget that if your seed does not say ‘low-fat ‘, then it is not 'low-fat', and if it does not give the fat content on the information provided, then there is something being hidden from you. Avoid avocado, onion, chocolate, caffeine and salty foods, all of which are toxic to budgies. Never feed 'honey sticks' or seeds stuck together with glue into a block. Keep it natural, a treat is a corn on the cob or a stick of celery or some seeded wild grass.
For cockatiels, they enjoy a much bigger range of fruit and veggies and their seed does not have to be as low in fat as budgies as they do not suffer as much with fatty liver disease. Make sure they have variety.
For both types of bird, protein is essential, either in the form for hard boiled egg, or use Just Budgies Protein Booster. Birds are animals and ALL animal (and humans) require protein to survive. This is one area that is so often missed in a birds diet and is often responsible for poor condition, over weight and shortened lifespan.