Beginning with the primitive Sinosauropteryx, all theropods (a class of meat-eating bipedal dinosaurs including Tyrannosaurus rex) may have had feathers. In some, like T. rex, the hatchlings shed their downy feathers as they grew. In others, feathers were present throughout adulthood.

Feathers were one of the adaptations leading to flight in Archaeopteryx and, perhaps, Archaeoraptor. These two species had startling similarities to both dinosaurs and birds. The November 1999 issue of National Geographic included the statement, "We can now say that birds are theropods just as confidently as we say that humans are mammals." Birds really are feathered dinosaurs.

Defining a feather defines the bird and conversely a definition of a bird is not complete without mentioning feathers. In my biology classes we were told never to use the word "all" as some exception could probably be found. Nevertheless, all birds have feathers.

Feathers grow from the base (follicle) and, as the cells get farther from their source of nourishment, they die off. 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.