The Birdsong
The nature of the magical phenomenon named the Birdsong
What is the Birdsong, actually?
The birbs don't realize this, but the force they refer to as "the Birdsong" is a field of nanomachines that permeates Bird-Earth's atmosphere. What the birbs refer to as 'magic' is their ability to use this as a force.
Birbs have a store of nanomachines that are attuned to their bodies. If a birb were to leave the atmosphere of Bird-Earth, they would find the power of their magic drastically diminish; a birb using its own local nanomachines is a much less powerful magic-user than a birb that is leveraging the entire planet's system of nanomachines.