I shouldn't have put "one of the few ballads" in quotation marks. Those aren't Mackenzie's exact words. What he actually says is: "the few ballads that he can still sing he has retained almost by accident. 'The Blaeberry Courtship,' for instance ...", but I think that amounts to much the same thing -- Henderson could still sing a small number of ballads, and "The Blaeberry Courtship" was just one of them.