Brown spots on budgie's feathers

Hello all,

Attached are pictures of my little budgie boy, Octavio.


Recently I've noticed brown/black spots on his feathers that were not there before (around the feet). I've inspected them closely; they appear to be stains, not mites. Each impacted feather seems to have a couple or more tiny spots in various places.

Does anyone have any idea what these might be?

Some helpful information:

He is on a seed and fruit diet at the moment (with occasional proteins like eggs); I am still in the process of converting him to a more healthy pelleted diet.

I haven't noticed any change in his behaviour/temperament/food intake/water intake.