I definitely don't agree with the part about no more than 5% seed for granivores. Assuming you've got a bird who enjoys eating a variety of foods and enjoys eating veggies, I don't see why they can't be fed a lot more seed.
I free-feed my birds because I have a mixed flock, and the different species require different ratios of each food. So, they have access to pellets at all times (which they all enjoy eating), a variety of vegetables placed throughout the aviary (well, I've been lazy lately, and I need to go to the grocery store to buy a bit of everything), as well as several handfuls of millet placed in foraging toys and thrown on the floor for them to forage. I also throw a handful of nuts and sunflower seeds on the floor for Noah and the kākāriki. They could easily choose to eat nothing but seed, but even the budgies will only eat a couple of grams of seed each after first choosing to eat pellets and greens. I also keep a dish of dried food-grade flowers (make sure they're intended for ingestion, as most dried flowers have harmful preservatives in them), a thing of soluble and insoluble grit, kelp granules, a Manu mineral block, and a blend of herbs in there for them.
While this is the best way to feed my guys, I'm sure that other birds would probably end up getting sick due to only eating what they want. My budgies and kākāriki (and Noah to a certain extent, since he's the guy who was trying to eat dried krill and dog food) seem to know their bodies and choose how much to eat of certain things depending on their nutritional needs at the time. Also, they live in an indoor aviary and are fully flighted, which means they're far more muscular and active than the average parrot in captivity. However, I definitely wouldn't trust larger parrots, or parrots whose diets vary greatly throughout the year in the wild due to food availability. I think when kept in a flock, parrots are more inclined to eat a healthier diet since they're copying everyone else, whereas a singly housed bird will just load up on fatty, unhealthy foods.
Kind of got off topic... Anyways, I think an improper diet CAN lead to a cranky bird. If they have too much protein in their diet, this can trigger hormonal behaviour. Too much sugar can lead to candida, which can make them feel tired and cranky. Not to mention that some birds are addicted to certain foods and refuse to eat properly, resulting in tantrums if they don't eat the food they want. They can actually start exhibiting symptoms of withdrawal due to the yeast and bacteria in their body demanding more carbs or sugar. If the carb and sugar levels don't return to "normal", the yucky bacteria and yeast suddenly die off, making them feel really sick. But, once they're flushed out of their bodies, they actually start feeling really good and have more energy. Or at least this is what happens when humans have to go on a special diet when they suffer from candida.
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