It is very important to understand that there is a vast difference between Human and Parrots and what is good for us may not be good for them. As both belong to different ecosystem and their needs and requirements vary from humans.
So to be a successful Aviculturist one has to be a good Ornithologist. So that its helps one in understanding the Parrot as it lives in the wild and provide an environment and diet as near as possible in their wild habitat.
The most common problem with an Aviculturist is that he tries to enforce principles that are good for humans forgetting that their needs are different and that they belong to a different ecosystem.
I am neither an Aviculturist nor an Ornithologist. I am just an ordinary Hobbyist trying to follow and apply both the principles for a better management the Parrots in capitivity. There are some doubts that I would like to have it cleared. So please share your thoughts.
The parrots in captivity are either under nourished or over nourished. 
 1. Undernourished – Nutritional deficiency, due to wrong diet. For example Macaws need high fat diet, Cockatoos need moderate fats and the Amazons need low fat diet. All seed diet is not good for parrots as it lets to nutritional deficiency as they lack in essential vitamins and minerals.
2. Overnourished – Too much of rich food can cause obesity, fatty liver and fatty tumors to name a few.
The very basic thing to understand is that no cage is too big for any parrot. But in captivity one has to compromise with the cage size, as it is not possible for everyone to provide large cages due to space restrictions. 
Many keep or breed parrots in small cages and proudly share their success stories. I have observed that Parrots breed better in small cramped cages, especially the African Grey they thrive and breed better in stressful conditions but not for long, but no one talks about the harmful effects as such conditions gives them good results.
Most of the Avicuturist are very keen in provide the best amenities to the parrots, when it comes to the design of the cages / Avaries, Nest boxes and location. But unfortunately it is to their liking rather than design them with the bird in mind and from the bird’s point of view. 
Many provide huge nest boxes; some of them look like a small cage inside a large cage. The nest boxes look like Penthouses. One of the reasons for this is that they do not want the tail to break or go out of shape; they also want the bird to feel comfortable inside the nest box. But they ignore the fact that huge nest boxes lead to broken eggs, chilling of eggs and chicks and trampled chicks. 
Parrots do not need huge nest boxes, and they cannot find a cavity in the wild that is in the size of the nest boxes that are provided in captivity. Parrots feel secure in small compact cramped nesting holes. The entrance holes provided in captivity are like huge gates to a castle. This most often the cause of broken and crushed chicks in the nest box as they tend to jump onto them while entering the nest box. 
Some sometime back I read any article by tony Silva. Where he had mentioned that during his research trips in Mexico related to Macaws that they came across some nesting cavities that were so cramped that the chicks were stuck in the nest box unable to come out and were being feed by the parents. They were removed from the nest after human intervention. 
Parrots always prefer small entrance hole leading to the cavities, into which they squeeze through. This give the parrot a sense of security that a predator large than them cannot enter their nesting site and that they can fend off any small predator that enters their nest. This small entrance and cramped nesting cavity slows the movement of the parrot while entering into the nesting cavity, thus reducing the chances of breaking, chilling off eggs and chicks, and crushing of chicks.
It is very important for healthy management of parrots. It should be a well balanced diet. Keeping in mind their wild back ground. What is good for humans may not be good for parrots. Humans can plan what to eat, when to eat and what not to eat. But parrots have no choice they have to eat what they get to eat in the wild. Any good diet - fruit vegetables or seeds is good for them at anytime of the year. 
Planning the diet based on what is good for humans is good for parrots are wrong. A simple example Avocado is very good for humans but not so for parrots. It can kill the parrot. Diet of the parrot should not be based on human diet principles. Like what to have when to have. What is good and what is bad for a particular season. 
Papaya is said to cause abortion in pregnant women, so should we stop feeding them to our parrots during breeding season. But when one discusses about diet like salt, milk and bread there is a barrage of opposition, as to the dangers of feeding them. But it is unfortunate that no one talks about sugar. Do you know that like in humans sugar is a silent killer in birds? Do you know that parrots suffer from diabetes?
In the wild the parrots have to compete with other animals like example monkeys, rodents. So they have to eat them before they do, so most often than not they end up eating raw fruits which are not sweet and low on sugar. They very rarely get a chance to eat a ripe sweet fruit. So they have to what they get and cannot plan as what to eat and when to eat. 
But once they are kept in captivity, without knowledge of their dietary background and the chances of the bird getting affected by Diabetes. The owners feed then with the sweetest fruits that they can find. Just because we feel they are very tasty to us, the normal sugar range for parrots is between 400 mg/dl to 700 mg/dl. Diet high in sugar and Carbohydrates are said to be the cause. Diabetes in parrots can cause issues with Liver, Kidney and Pancreas. 
We should always provide them with diet, with their wild back ground in mind and as close to as possible, rather than what we feel is good for them.
We can only provide them with a diet as close to their natural wild diet. Because in the wild they feed on wild fruits and berries and not on fruits and berries we provide them with. I doubt they have access to many vegetables that we provide in the wild. Most of the parrots rarely come down to ground to feed. So the chances of digging into the ground to eat Carrot, Beetroot or Sweet potato are remote, so all that we give are substitutes near to their original diet. 
Most of the parrots in captivity are overfeed, which makes the obese and less active. They still end up with malnutrition due to wrong diet. Parrots eat somewhere between 20 to 30% of their body weight in a day depending upon type of diet they eat. 
Parrots often fly many kilometers in search for food.  Macaws have been recorded to fly 50 km looking for food. So the whole day is occupied in search of food. Resting in between, preening each other and basking in the sun ending up eating 20 – 30% of their body weight in a day.
In captivity there is no need to search for food and end up over eating as it is given to them in abundance all at once followed by two or three more portions. Which they consume within minutes, that which would take them a whole day to consume. And do not even fly a kilometer in a day in the cage or aviary, leading to obesity and other issues due to lack of exercise.
Improper diet and overfeeding parrots leads to breeding issues. The birds in captivity have abundant found ready available all through the years. Many reasons have already been discussed earlier. Rich found all round the year in a way hampers the stimulations of breeding hormones.
In the wild the parrot sex hormones a stimulated by the changes in the season and availability of food. They start breading when abundant food is available to them. The availability of fresh grains, fruits and vegetables stimulates the sexual hormones and brings them into breeding condition. 
Many parrots do not breed as they are stuck to a rich diet available all through the year. They do that go through the cycle of drought followed by harvest many leading Aviculturist follow two types of diet Non breeding diet and Breeding diet. 
In case adult birds are not breeding or stopped breeding. Putting them on non breeding diet died for a month or so will stimulate their breeding cycle. Put them on a all seed diet bland diet, slowly introduce them fresh fruits, vegetables sprouts one by one. They should surely breed. This has worked for me with Amazons, Conures and African Greys.