If by tail markings you mean white areas in the tail, it is not terribly unusual for birds of many species to have white patches in unexpectated areas due to partial albinism. Given that it is with house sparrows, the obvious alternative to consider is that it is a house sparrow with some extra white on it.
Also I've always felt that McCown's longspur is the one bird I'd pick that is least well portrayed in field guides - at least on the breeding male, what looks gray in the book is nearly silver in life.