 Joseph Jordania suggested that human humming could have played a function of contact calls in early human ancestors.According to his suggestion, humans find it distressing to be in full silence (which is a sign of danger for them), and this is why humans who are alone sometimes hum, whistle, talk to themselves, or listen to TV or radio during other activities.Birds use contact calls in flight to establish location and to keep aware of each other's presence while flying and feeding.