Birds which are actively breeding must be fed an appropriate diet. Foods formulated for "daily" or "maintenance" use will not suffice for long-term health and success. Breeder foods are enriched with more protein, more fat, and more micronutrients. This allows the parents (especially the hen) to safely reproduce and raise baby birds, and also the baby birds to grow and develop at a healthy, steady rate.
Seed alone is NOT suitable to feed breeding birds. This puts both the parents and the young at a high risk of malnourishment. This can have severe consequences including stunted development, egg-binding in the hen, and in some cases even death. If you wish to feed a seed-based diet a well-rounded vitamin supplement is needed. Fresh vegetables and some fruit are strongly recommended as part of the daily diet.
A generalized diet would be...
Outside of Breeding: 65% pellet, 20% vegetable and fruit, and 15% seeds and nuts
3 weeks Before, While, and 3 weeks After Breeding: 50% breeder pellet, 25% vegetable and fruit, and 25% nuts and seeds
This diet plan works well for most types of birds, however individuals and species may vary within their needs. Knowing about your exact species is extremely important when making a diet plan for the birds.