A voice box called Syrinx.

The voice box called Syrinx can be found only in birds, and it represents a rare example of a true evolutionary novelty. Yes, birds do have Larynxes as well as mammals, reptiles and amphibians, but the Syrinx is located lower down, where the windpipe splits to go into the the two lungs.
Crocodiles diverged from birds 240 million years ago, and we know about their calls, but crocs just have Larynx. If the Syrinx evolved as birds came into existence, then some dinosaurs may have had two sound sources, so probably one day there will be a fossil found of a dinosaur with a Larynx together with a Syrinx. 
This Three-wattled Bellbird (Procnias tricarunculatus) has the loudest call in the world, so probably its Syrinx it’s one of the most evolved of the avian kingdom. Enjoy this mix of a bell with a bird!