Beneath the horizontal line are:
Kiwikiu and Maui Parrotbill are simply the Hawaiian and English names for the same bird.
The main difference I see in these prints (beyond style) is the treatment of the beaks. Robert's beak is the stoutist, Frohawk's the longest, and Marian's is somewhere in the middle on both counts. A look at several photographs on the internet leads me to believe that Frohawk is closest to reality, but then individual members of a species differ. This seems substantiated by the photo by Jack Jeffrey.
Another difference is the more yellow coloring of the feather for Marian and Frohawk, with Wagstaff's depiction having greener tones.