I wonder if some bird expert actually checked whether or not birds speak with frequency-modulated words instead of our familiar amplitude modulation.
Checked, not assumed it was so obviously absurd someone checked negative already.
Listen to a bird chirping - the chirps are not at regular intervals. Maybe the offset from some basic rhythm is the encoding?
Have you noticed, that when you listen to a regular sound, you expect the next sound happen, and if it don't, there is an expecting tension?
So, less tension is -1, regular is 0 and more tension is +1.
All that is left is to decode this trinary morse code and you can talk to birds.
[/absurd_mode]