I couldn't find an answer to this myself - is there any reason why the preening gland on birds is located on the tail and not elsewhere on the body? Why did it evolve to be on the tail and could it have been located elswhere on the body?

Thank you.

the simple answer is no-one knows why it evolved to be located at the base of the tail. it is possible there was some advantage to that position which is now less obvious. Alternatively the location may be unimportant and as you imply it is just where the early bird's gland was located and there has been no evolutionary pressure for it to move.