Those look like stress bars to me. As the term implies, they are usually an indication of some kind of stress during the time of development of that particular part of the feather. The stress could either be diet related, environmental (extreme temperature shifts, changes to his home setting, or the physical stress from a lack of sleep) or even psychological (such as consistent or acute fear, for example.)
Depending on when he last molted, the stressor(s) could either be in his current environment, or in his past home. (If there was a past home) It could even have come about during his transition from one home to the next.
Bixby came home with quite a few, but he has grown most of them out at this point.
What is his current diet? And are there any other potential stressors in his environment? How long have you had him?