I recently adopted a gray male cockatiel, about 2 years old. I found out that he has this white patch of pin(?) feathers underneath his wings. The patch on the left is bigger than the patch on the right. You couldn’t tell visually that there’s a patch there, even when he lifts up his wings. Really had to dig through his feathers to find the patch. But once you see it, you realize that it’s a really big patch that’s covered up by his feathers. Is this some sort of plucking problem? Or is it a type of bacterial infection?? 
