Listen to the sound produced. Stretch the neck of the balloon more or less to produce different sounds. Birds are born with the ability to sing, but they must learn the songs of their species. If a baby bird is isolated from other birds of its species, it can instinctually sing simple songs, but not the more complex songs of its species. These it must hear and learn.
Most birds can only sing the songs of their species, but there are exceptions.
The mockingbird seems to be the best at imitating the songs of other birds, and captive parrots and mynahs can even imitate human speech and other sounds. Birds have an organ called the syrinx in their windpipe that enables them to create sounds. The number of muscles around a bird's syrinx determines the variety of sounds that it can produce. In general, the more muscles surrounding its syrinx, the wider the variety of notes a bird can sing. The syrinx is located where the windpipe divides into 2 tubes, one going to each lung.