What is a light thin growth covering a birds skin?

A light, thin growth covering a bird's skin is typically referred to as feathers. Feathers serve various functions, including insulation, waterproofing, and aiding in flight. They are unique to birds and are made of a protein called keratin, which also forms human hair and nails. Feathers come in different types, including contour feathers, down feathers, and flight feathers, each serving specific roles in a bird's life.