Mockingbird practicing repetoire. 
Near the end of video, the show-off does an aerial stunt, then continues singing. (at 1 minute in)

Full Screen entertainment: You can occasionally see the bird open beak as his throat puffs out the audio.  That is the sound  emitting hardware.

That is new information for me.  I know birds do not have vocal cords (How humans make sound).
So how do they make such a loud sound?
Now I know.  More evidence of Intelligent Design!