Defining a feather defines the bird and conversely a definition of a bird is not complete without mentioning feathers. Nevertheless, all birds have feathers. As the cells die, they become filled with a horny substance called keratin. In scientific lingo, a feather is simply an elaborate and specialized keratinous product of the epidermis of a bird. Hollow tubes of keratin, like hollow tubes of modern day aluminum, are very lightweight and very strong.
Feathers come in many designs.
Some are used for insulation, some for flight, and some for protection. Humans have utilized the insulation characteristic of feathers for generations and in recent years have tried to mimic the benefits with manufactured materials. It still remains difficult to duplicate the efficiency and warmth of your Grandmother's feather comforter.