The vocal skill of birds derive from the unusual structure of their powerful vocal equipment. The syrinx is the sound-producing organ in birds. It is the equivalent of the human sound box. The syrinx contains membranes which vibrate and generate sound waves when air from the lungs is passed over them. The muscles of the syrinx control the details of song production; birds with more elaborate system of vocal muscles produce more complex songs.
The syrinx has two potential sound sources, one in each bronchus. The separate membranes on each bronchus produce separate sounds, which are then mixed when fed into the higher vocal tract. This complex design means that birds can produce a far greater variety of sounds than humans can. A bird can sing two different notes at the same time, or even sing a duet with itself.
syrinx

 

Birds give the impression of singing in long bursts for minutes on end without catching their breath. But they actually do this by taking a series of shallow mini-breaths,which are synchronized with each syllable they sing.
A human singer can use amplification to hide his or her vocal deficiencies. But bird song conveys a very honest message about the singer. The singer can't cheat: because singing expends energy; smaller, weaker birds cannot bluff the receiver into thinking that it is a larger, stronger rival or mate. Only strong birds with extra energy and strength can invest the energy needed for loud, continuous singing, and evade any predator that may detect it.
Imitating human speech is a difficult trick and parrots and their smaller kin, the budgies, use different techniques.
Parrots have thick tongues like humans, a novelty among birds, which has the side effect of allowing them to form the
sounds of words in a manner similar to us. That means that they emit a preliminary sound from their syrinx and then modify it using the mouth, throat and tongue.

The best time to hear bird song is at dawn. The dawn chorus is one of the marvels of nature. Birds all over the world show the greatest amount of singing activity around dawn. One reason may be that dawn is the best time for sound to travel, because there is little wind and less other noise and disturbance.

 
Content Courtesy: www.learnbirdsongs.com