RavenLite, and when I saw this in the "twee!" syllable of Pine Siskin song, I realized something interesting was going on. Not only were there two independent signals, the melodies were going in opposite directions, and the composition led to a great deal of harmonics. The most interesting thing is the way you can tell that they are harmonics: the slopes of the lines increase geometrically as they increase in frequency, causing what looks to be an exponential increase.
Who says songbirds aren't fascinating?!