I accept there are different ways to raise each animal but this one fails to make sense to me. Birds learn to survive and be "strong" by interacting with their environment. The wood, the leaves, manipulating sticks, and finding bugs. Standing on different types of footing and different widths to exercise and strengthen their feet. They don't get strong and survive by sitting still on a perch. They also start out interacting and forming relationships at hatching and become stronger survivors by knowing their flock which is replaced by their human caretakers. Hatchlings have been shown in many species to recognize their parents' "voice" and the parents their offspring from a flock all making the same type of call. They form social bonds with the rest of the flock as soon as they leave the nest. Training is a mimic of that bonding they do from day 1 rather than leaving them isolated until they get older which is against nature. Toys and perch variety are a mimic of all those objects that build their mind and bodies from the day they leave the nest. They would normally be in a place just full of things to grab, peck, and carry around. Many people even have things they can hide bugs or vegetables in, depending on what their species eats, to copy foraging/hunting behavior. They exercise themselves figuring out where the food is and how to get it. That can also be somewhat a social/training moment because smart birds will see you filling the foraging toy or putting it in their cage and know you provided them with tasty food and entertainment so they are happy to see it and you. Usually it is believed that the smarter the bird the more mental exercise it needs with toys and foraging to not turn aggressive toward other living things and grow up to be mentally or emotionally healthy individuals that interact well with their humans. Some birds lacking in objects to play with, humans to interact with such as out of cage time and training right away to come to you and step up for treats, or other birds around will start pulling their own feathers or biting themselves in frustration and boredom from their loneliness and are more likely to attack their handlers for the same reasons.