'Sympathetic Song': the Silent and the Overt Vocal Repertoire, Exemplified with a Dueting Pair of the African Slate-Coloured Boubou, Laniarius funebris

In many bird species the total song repertoire consists of an overt repertoire normally uttered, and a silent repertoire produced only under special circumstances. A silent repertoire can be inferred from specific vocal responses of an individual to hearing parts of its own silent repertoire uttered by another bird ('sympathetic song'). The silent repertoire plays a prominent role in dueting birds, in particular those with sex-specific overt song repertoires, where the overt repertoire of one partner is the silent repertoire of the other. This is here exemplified with a pair of the slate-coloured boubou L. funebris . Silent repertoires should be taken into consideration when relating the complexity of birds' vocal behaviour to the size or structure of their forebrain vocal control areas.