Hello guys! I found this site in interest of asking a question concerning my bird. What are the causes to a dirty butt, besides stress and diarrhea? My bird has neither of those.

I have an older male Rainbow Bourke Parakeet, we got him from a breeder years ago and his cage has been kept the same for the longest time. The last time we switched up his perches was weeks ago. No new changes in the house, havent moved, theres no reason for this to be because of stress. His food has remained the same, as well.

A little while ago I noticed his butt was a little dirty, but we just kept clean bath water in his cage and waited to see if it would go away.

Finally got to cleaning him up yesterday, and I've noticed not only was his butt dirty around the vent, but he also has a bald patch around it/slightly up on his belly. It looks dry and a little flaky. I've attached pictures, although they were after I cleaned him up.

Quigley, the bird, is as normal as ever. Eating, drinking, bathing, no differences in him at all.
Before telling me to ship him off to the vet, I'd rather do my research first and figure out what might be the cause of this, and if it's something I may be able to treat at home, if there's something wrong with him at all. I'd just like to do some research on it so that way I can care for Quig to the best of my ability.

I tried doing some research and I'll keep looking while waiting for answers here.
Thanks!