Contrary to popular belief, birds do not live on seeds alone! Birds have a habit of picking out the kinds of seed they like and ignoring the rest, so they can't get a balanced diet from even the best seed mix. That's why researchers have developed a number of "pellet" foods (available at most pet stores), which contain balanced nutrients in every bite.
Many birds look at pellet foods suspiciously at first, so don't try to "convert" them from a seed diet to a pellet diet all at once. Follow the conversion instructions on the package carefully, and don't give up! The right food can add years to your bird's life...and improve its overall health and appearance.
In addition to pellet food, offer your bird a variety of fruits, vegetables, cooked beans and rice, raw leafy vegetables such as collard greens and kale, whole grain, dry, sugar-free cereals. These foods should be placed in a separate bowl (not the pellet bowl) and removed within two hours so they don't spoil. Depending on how it was raised, your bird may not accept these foods immediately, but keep trying. Some birds refuse to eat these foods out of a bowl but will accept them if they're hung from the side or top of the cage. (You can buy special clips and skewers made for this purpose at your pet store.)
Birds that were hand-fed often take nutritious foods from a spoon or their owner's fingers. In fact, sharing safe treats with your pet is a great way to strengthen your friendship.
Do not feed your bird even small amounts of avocado, chocolate, rhubarb or alcohol -- these foods are highly toxic to birds. All sugary or salty foods are also harmful to birds.
Every bird should have a cuttlebone and a mineral block at all times. If you have a budgie (parakeet), make sure its mineral block contains iodine, which is essential to their health.
Remember to give your bird fresh water every day. After you wash and thoroughly rinse its water bowl, let the cold water tap run for two or three minutes. This cleans out bacteria and some of the lead that accumulates in older water pipes overnight.
Certain species, such as lories and lorikeets, have very specific dietary requirements. Consult your veterinarian for nutritional advice specific to your pet.
More Reading on Diet and Nutrition:
Avian Nutrition - Psittacine dietary requirements - Dr Adrian Gallagher; PSA
Birds Don't Really Eat Like Birds - Krista Menzel; Springville
Converting Seed 'Junkies' to a Better Diet - Monica Sudds; Beakers
Dealing w/ Vitamin A Deficiency - Hannis L Stoddard,III, DVM; HotSpot
Diets & Treats FAQs - at Up At Six
The Maize Craze - Carolyn Swicegood; EE
Metabolic Energy of Sunflower Seeds...and... Composition of Oil Seeds - HARI
Nutrition: Foundation for a Healthy Bird - Susan Schwab, DVM; HotSpot
Nutritional Psittacine Diet (fresh food) - Alicia&Bruce McWatters; SPBE
Parrot Nutrition - David Poole
Spirulina - Wonder Food For Birds - Ronald H. Henson; Kim's, BirdWorld
Supplementing Nutrition with Sprouts - Geoffrey & Barbara Gould; PPS & CBH
What About Protein - D. E. Ullrey, PhD; Kim's
What Do I Feed? - Monica Sudds; Beakers
Wheatgrass Analysis - BeakersAvian Nutrition - PJ Schimel; Winged Wisdom