When a bird’s beak begins overgrowing it’s usually a nutritional problem. It’s especially seen in budgies/parakeets and cockatiels. More commonly occuring when there’s presence of malnutrition as the result of a largely seed only diet.
Often there is also liver disease as a result so the overgrown beak is a symptom you shouldn’t ignore.
See more about these problems here:
http://www.peteducation.com/article.cfm?cls=15&cat=1829&articleid=2752
It’s generally recommended that most of today’s companion birds have a predominantly pelleted diet. Pellets have been continually updated since being introduced to the market years ago and today’s formulas are better than ever.
Supplementing this diet with fresh foods every day is ideal and many owners find they can re-introduce seeds - in limited amounts (perhaps once or twice a week) without the bird refusing the pellets overall.
Whole grains, dark leafy vegetables, fruits and legumes. Include the colors orange, yellow , green, plus reds too! Think sweet potatoes/yams, squash, melons, oranges, peas, chard, beets and others.
Brown rice, quinoa, whole wheat couscous and natural, whole grain pastas are great choices.
Limit fats, especially the kind from animals. Good fats are most plant fats like soy, olive and canola oils. No fried anything
Another thing you can try is all natural, human baby food. Stick to the orange colors.
They can be mixed with tiny pasta or rice, whole grain bread or toast - remember, be more creative than the bird is stubborn.
Even though this is put out by a cockatiel site, it’s applicable to all hook bills from budgies/parakeets to conures, greys and macaws.
http://www.cockatiels.org/articles/nutrition/diet.html
Certified Avian Specialist
Cert. Avian Specialist; Int. Assoc.Animal Behavior Consult; Pet Ind. Joint Advisory Council; author