The story of bird song began some 150 million years ago when ancestors of modern birds evolved feathers and the ability to fly. These adaptations allowed birds to disperse, find food sources, and
find safe haven in dense vegetation. The new avian life style favored vocal communication to keep contact over long distances or while out of sight.
However, birds are at risk when singing, since predators might hear, locate, and eat the singer. Countering that threat, birds' mobility allows them to escape, and they can sing from hidden
positions, and even in the dark. Judging by the evolutionary success of birds, the advantages of vocalization far outweigh the risks.
One ancient lineage which includes ducks, geese, chickens, and quail illustrates the fundamental nature of bird vocalizations. Each of these species has a vocabulary of roughly 10 to 20 calls and
Calls are short notes, like saying "Hey" -- chips, chinks, quacks, and high pitched sounds such as "peep" and "seeet". When a chicken clucks, that is a call. Birds call when alarmed by a predator,
to rally or assemble around food, when courting, and as youngsters begging for food.
Songs are longer phrases which typically have more of what we think of as melody. Songs are usually associated with breeding -- attracting a mate and defending against rivals.
When a rooster crows, that is its song. (The call versus song distinction is fuzzy since "songs" are sometimes sung outside breeding situations and "calls" may also be used in breeding situations.)