A study of northern cardinals found that their songs sound sweet because they can modify their throats to control overtones. When we watch singing birds, we can see that their throats vibrate with each song. X-ray images reveal that the outward vibration is caused by the throat expanding and contracting with each note.Overtones surround most sounds made in nature, including birdsong and human singing. Their volume relative to that of the fundamental pitch results in a sound that can be pure, rich, or muddled. Reducing the relative loudness of overtones will result in a purer sound.
Make sure to watch the include video clip, which shows the throat in action.