Why do sparrows shake their heads while eating at a feeder?

Sparrows shake their heads while eating at a feeder to remove unwanted husks and debris from the food. Many seeds have a hard outer layer that must be removed before the seed can be eaten, and sparrows use their beaks and necks to shake off these husks. By shaking their heads vigorously, they are able to separate the nutritious part of the seed from the inedible parts.