Yamaguchi sees cardinals as essentially a tropical species, which spread north only during the last hundred years. A male and female cardinal sing the same types of songs, and to people, her version sounds very similar to his, Yamaguchi observes. Yet the birds know the difference. She has found that male cardinals attack speakers broadcasting a male song, and a female recording prompts females to sing. After listening to the songs for about a month, Yamaguchi thought of comparing their harmonics.
Female versions have more overtones, creating a nasal sound much as bagpipe overtones do.
Also, females do not repeat notes as precisely as males do. Yamaguchi rejects the idea that the females learn only from Mom and males just copy Dad. Thirty cardinals raised in captivity learned songs from recordings of both males and females, she reports. Likewise, males added guy traits to songs they picked up from a recorded female. Hormones might explain the song differences, Yamaguchi suggests.