I think the mystery has a simple explanation: when a bird sings at 7 kHz and the mp3 file captures only first 20 kHz, there isn't much room for harmonics. Maybe

I think the mystery has a simple explanation: when a bird sings at 7 kHz and the mp3 file captures only first 20 kHz, there isn't much room for harmonics. Maybe birds do have interesting harmonics at 56 kHz, we just don't know.