In the wrong song example ... A male cactus finch can learn the medium ground finch song. In a case where, a male finch does learn the wrong song. When the father dies and they learn the song of a male neighbor of a different species.

If a male finch learns the wrong song, it might mate with the wrong species, provided that their size and overall appearance are not all that different. The offspring of a cactus finch and medium ground finch would look like an intermediate between the two, but the song would not be an intermediate, it would be the same song the male parent sings!

These hybrid offspring are rare, and they are not as fit as either parent.
Can you think of why that is?