Co-written by Lola (Frank was too busy to help.)
If other food is always available, why try something new? Similarly, suddenly taking out a dish that is always available often just leads to a hunger strike. Birds in the wild do not feed all day, and as ours are getting far much less exercise than them, I see no reason why they need food all day, either! Two meals a day, with some treats hidden in toys if you like, is a more natural way to feed. Watch the birds outside your door- heaviest in the morning and evening. Allowing the stomach to fully empty in the afternoon is not just natural but healthy- it allows the body to cleanse itself, heal, and re-set for the next meal. Feeding two meals a day makes food more valuable, more exciting, and can, and often does, help your bird gain weight, and by monitoring the food to a healthy amount, can also keep their weight down- which for parrots is even more important than for us. And since food is more exciting and valued, they are often more willing to try something new.
I say get to two meals a day with your current diet. Decrease or increase the amount of food at each meal until it is just enough that there are a very few crumbs left. If they were still starving, they would clean up those crumbs, but they still qualify for the clean plate club by only leaving a few crumbs. As I said before, if you want to fill foraging toys, take some of their meal amount to do that- ie, do not feed two full meals plus many treats (even if those treats are very healthy), but include the treats in the amount of food they can eat in one day. Once they are eating all of their food, try adding new food at one of the meals, or both. Not cold turkey, but mixed in, using still the same amount of food they can eat. While doing this, do monitor weight, if they lose more than 10 percent, back off. I try to do it without much weight loss at all, although sometimes there is no way to avoid it, just like leaving your child without dinner (been there,survived, and eat everything healthy I can!)
This may not help some of you, but I yet to find a bird that it did not widen the diet at least the tiniest bit. And even if you are not trying to get them on a new diet, I still recommend the two meal system, for all the above reasons. Trust me, outside of some severe health reason, your bird will not suffer, but be healthier!