Wanderers are birds that don’t fit any of the other categories.
The term includes birds that made what appear to be navigational errors of some sort, overshooting their breeding range or migrating in the ‘wrong’ direction. Some are displaced by irresistible forces like powerful storms. A few are so far out of range, many thousands of miles in some cases, it’s hard to know where they went wrong. We can’t predict, other than in a few cases and in the most general terms, when wanderers will show up again or how long they’ll stay.