Aeolian Harps - brilliant! That would explain the small number of strings and the ability of the harp to play by itself, as well as the small size necessitated by using a human breastbone.

Incidentally, Aeolian harp kits are available from several sources and I have also seen instructions for building them in a woodworking magazine - for those of you who want some chordal accompaniment for your wind chimes. These days they are placed in partially opened windows instead of hung in trees.