A parrot fed only sunflower seeds is eating the bird equivalent of chips for every meal. It is high in fat, low in most of what the bird needs, and the results show up slowly -- dull feathers, fatty liver, a bird that dies years earlier than it should.
Better, roughly in order of how easy they are to get here:
- Fresh vegetables -- carrot, spinach, peas, capsicum, cucumber
- Fruit in moderation, because it is mostly sugar
- Cooked pulses and boiled rice
- Sprouted seed instead of dry seed
- Pellets, if you can find them and if your bird will accept them
- Cuttlebone or a calcium block
Never: avocado, chocolate, caffeine, alcohol, or anything salted.
The hard part is that a seed-addicted bird will genuinely go hungry rather than eat a vegetable. How did you convert yours? Mixing, chopping small, eating it in front of them, offering the same thing for a week straight -- what worked?