Birds do not have vocal cords. The more muscles a bird has attached to the syrinx, the more sounds it can make. A variety of other types of communications are used by birds. Hungry nestlings peck at their parents' beaks or open their mouths widely to beg for food. Sandhill cranes and American woodcocks have elaborate mating dances and flights.
Communication is very important to birds.
Without communication, many birds would starve, lose their way during migration, be unable to defend a territory or find a mate.