Peter and Rosemary Grant found that the medium ground finch (Geospiza fortis, #13) and the cactus finch (G. scandens, #3) are capable of interbreeding.
 
The two species remain separate largely due to oblique and vertical transmission of songs that allows females to recognize males of their own species and provides a barrier to forming hybrid offspring.
 
Males who learn the song of another species often can breed with another species, resulting in hybrid offspring.