Immature feathers are called pin feathers or blood feathers because they are still living tissue. Because they are growing so fast, pin feathers contain a great deal of blood, which gives them their dark, bluish color.
Feather grows requires a constant blood level of nutrients. If a baby parrot is off feed for even a half day during the time its feathers are forming - you will see a "stress bar" (fault bar or fault line) on the feathers that were most actively growing at the time. Improper temperature and other stressors can also lead to this problem. Direction of stress bar can go length wise or across feather.
This is a little line across the feather - as if it had been scored, marked or cut with a scissors at that point. The area of the stress bar is weak, translucent and easily torn. When multiple bars form, each bar represents a period of stress. It is quite difficult to hand raise baby parrots without a few stress bars forming. In subsequent natural molts, normal feathers will replace them. But when they occur in mature birds, they are warning bars. An avian veterinarian needs to be consulted to determine the underlying cause.
That's what I can come up with from my wife's Merck manual of vet medicine.