I have just noticed my budgie has a dark coloured growth approx 1cm in length, just above his tail feathers. I fear this is serious, we do not have a vet in our northern community.
Yes, it may be serious. Dr B can not tell you, because he can't examine your bird or run tests on the growth. You will have to make the drive to an avian vet to solve this problem.
There are no over the counter solutions to growths, especially when you don't know what kind it is or what is causing it.