On the first official "Arbor Day", people in the State of Nebraska planted more than
trees.

Explanation:

The song is learned. Experiments have shown that if a young male bird never heard the song of its own species when it was growing up, then it won't be able to sing the complete song that its own species normally sings. Learning is also one reason many songbirds have regional dialects. Morphology OTOH is determined genetically so it is intermediate because the hybrid have genes from 2 different species which have different morphologies. So, the hybrid would learn either one song or the other, or perhaps even both songs, but not an intermediate song...