“Facts swooped like swallows, darting across her mind; there was a rush of pride in things still remembered. Singing was limited to the perching birds, the order Passeriformes. Nearly half the birds in the world didn't sing, but they still used sound to communicate- calls as opposed to song. Most birds had between five and fifteen distinct calls in their repertoire; alarm and territorial defense calls, distress calls from juveniles to bring an adult to the rescue, flight calls to keep the flock coordinated, even separate calls for commencing and ending flight. Nest calls. Feeding calls. Pleasure calls. Some chicks used calls to communicate with their mothers while they were still in the egg.”
Book:The Gravity of Birds