Symbolism, Imagery, Allegory
This blue-black bird with a yellow bill is smarter than it looks. A Matsika family pet, it can be trained to speak. Can your pet do that? We didn't think so. (What's that? You have a parrot? Fine. You win.) Yet it's not the bird's mad communication skills that make it so interesting to us—it's the fact that it's trapped.
Just like Tendai, the mynah wants to be free from its cage. It's sick of being confined. The bird symbolizes the feelings that Tendai has about his life—so he releases it. If he can't be free, at least the bird can be:
The mynah lifted on a rise of warm air as it coasted over the garden wall, over the snapping electric wires and broken glass and machine guns. It was a black dot in a hot blue sky, going swiftly, and then it was gone. (2.86)
Tendai can almost feel the wind beneath his wings. It's not until he releases the bird that it dawns on Tendai that he's unhappy being trapped this entire time, too. Before long, he sets out on his own adventure, and in this way, the bird's departure foreshadows his own.