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.