Nutrition is a hot topic, i.e. one with many strong opinions and no final answer or widespread agreement. One thing seems clear, the all seed diet is a death sentence. I no longer feed seed to my birds, as contaminated seed has been implicated in bacterial infections in birds. Seeds are promoted by the same people who push birds as low maintenance pets. Birds eating only seed are going to be at risk of fatty liver disease (seeds are loaded with fat) and are getting no Vitamin A (one of the most common health issues vets see).
Feeding a high quality pellet (such as Harrisons) as the foundation of a birds diet is the best approach we now know of. My birds sometimes get another brand of pellet (Zupreem in natural color). You won't find Harrisons at your favorite pet store, you will have to get them from your vet (and they are expensive) or order them. The dyes known to color pellets have been shown to cause trouble for some birds (yes food grade dyes can be bad for birds), so I will never feed colored pellets. Also any pellet that relies on animal protein as a protein source should be avoided. An overload of animal protein can cause kidney failure, particularly in parrots.
Birds also need lots of vegetables and fruits, served fresh every day! And notice that we mention vegetables first. The best vegetables are the dark green leafy ones. Cooked sweet potato is full of vitamin A, as are many melons and squash. Many birds get sick because they don't get enough vitamin A, so give them plenty of these.
And think variety. Both for the intellectual stimulation and the nutritional benefits. If I offer bananas several days in a row, they will often be ignored the second or third days. A varied diet is as good for them as it is for us. And we can learn a lot about good nutrition by trying to feed our birds right.
And avoid these things as harmful or deadly:
Here are some links if you want to read more on nutrition:
Uncle Tom's parrot pages / tom@mmto.org September 27, 2003