Hi,
When ever a bird starts over preening, or plucking. You start with an avain exam, to rule out health issues. Fix tge health issues fixes tge problems.
If its behavior. Then I recommend a whole burd approach.
Make sure cage is large enough. Plenty of easy to shred ir ripped apart stuff, like yucca chips, those birdie donut ( looks like a paper ring) shredders are woven natural material thst you can weave in the cage bars. Yiu can even tear out dome pages from sn old paperback book and weave in cage bars fir them to shread
Make the aim 6 hours of out of cage time , thats from my CAV when any behavior issues. Some if that time Yiu are focused on them interactions with them, the rest csn be hanging out near you, or in play area
.n
Burds are flighted monkeys, they also like to climb, swing, dangle, fo daring leaps. So think of providing a jungle gym like for them. The easiest way is turn the outside if the cage top and sides with sea grass cargo net, perches, bird safe ropes ect. Then use that space above their cage! I out in ceiling hooks and used fishing line to hang and create all kinds of fun with swings, hoops , ladder, sea grass net, spiral perches and hang dome just st head hight above cage do they can grab and climb. Attach little things for them to find and discover, chewable, toys, treat sticks, millet. This really engaging to their minds, fun for them, and increases activity and reduces stress. Improves their self confidence too.
Offer a wide shallow dish, I use casserole dishes, with 2 inches or less of water daily fir them to choose when to take a bath. Splash in it with your hand can really bring them over and encourage a bath. Some days switch it up and mist water always pointing up snd allow to fall down on them. If they are in the mood give a good misting, if not dont force.
Taking a parrot outside in the shade. In travel cage or secure all openings on their main cage, including bottom grate and food dishes openings, and role outside. Stay with them always. Time outside helps them get vitamin D , even in shade, light shade, improving their mood, behavior, and feathers. I try fir twice a week. Or you could take them on a walk or bike ride in travel cage
Outside of the cage mine are more willing to explore and try new foods. Get those burd skewer and put on broccoli, or other veggies, suspended at birds chest height. When they come across this the urge to st leat rip it up is strong. Aldo spread out veggies in very shallow wide dishes or plates on top of cage. I keep seeds and pellets in seperate dishes.
A goid practice csn be offering veggies first thing in the morning, having pellets always available, and offering seeds in late afternoon. Also feed seeds as treats by hsnd during the day, for good behavior, for being quite, fir coming over abd saying hi, for stepping up. Use to target training to st least touch toys. Ect.
Have a scale and monitor weight daily. You do nit want to cause weight loss.
Their are lots if info and different techniques to diet changes out there.
This is great article about stress in parrots, and yiur burd seems to be displaying stress
https://lafeber.com/pet-birds/stress-reduction-for-parrot-companions/