The sound of Bertie's and ED-E's singing are enough to draw Wander's attention, even if he never heard the melody before. And obviously, since it looks like it's starting to become a group song, he knows he has to join.

He runs out of the room for a minute, as his room is just down the hall, and returns with his banjo, already tuned. Without interrupting the duo, he softly strums several chords until he's confident with the harmonization, then goes straight to providing accompaniment.