Human: I've heard that a hummingbird's wing is transparent. How can that be? Assistant: 
The wings of the hummingbird are made up of thin layers of cells, called feathers. Each feather is composed of two layers of keratin (a protein). The outer layer of each feather is called the rachis, and it’s responsible for giving the feather its shape. The inner layer is called the barbules, and they give the feather its color.