NARRATOR: All Darwin's finches, like fortis and scandens, are similar in plumage. All build their domed nests hidden in cactus. All have similar courtship behavior, often using a white seabird feather. They differ in body size, beak size and shape, and in song. It is song, in association with the bird's appearance, body size, and beak dimensions, that is a pre-mating barrier to interbreeding between the species. [FINCH CALLS] Fortis has a blunt beak, scandens a sharp pointed beak, magnirostris a massive beak. [FINCH CALLS] Fortis songs, on the left, are short and modulated. Scandens songs, on the right, are a series of repeated notes. All individuals sing one short song. There is variation between individuals, but always on a species-specific song theme. The fortis song above sounds like this. [FINCH CALL] A more rapid variation from another fortis individual sounds like this. [FINCH CALL] Scanden song, with repeated notes, like this. [FINCH CALL] The scandens, below, has a compressed song like this. [FINCH CALL] It is not difficult for us to tell the species apart using size, beak shape, and song. But can the birds? To test if a male territory owner can discriminate between its own and another species by appearance alone, we put two stuffed museum specimens, a female fortis and a female scandens, at either end of a pole. The answer was a clear yes, as demonstrated in this old Super 8 video, taken on Genovesa with two different species of ground finches. The female of the male's own species is on the right. The female of the other species on the left. The male territory owner flies up, moves left, and then towards the female of his own species, courts briefly, and attempts to mount. He flies down, picks up a white seabird feather, courts again, and mounts. We then asked if an individual could distinguish between its own and another species purely on the basis of song, in the absence of any morphological cues? Again, the answer was yes. Playback of previously recorded scandens song elicited a response from scandens, but not from fortis. Likewise, fortis responded to fortis song, but not scandens song.