Didn't you ever notice the dots all over the skin of a chicken?
Each dot is a little hole in the skin where a feather sprouts.

It grows in a sheath that pushes out of the hole, like a plant coming up out of the ground from its root.

For a while this sheath is full of blood to nourish the growing feather; that is why new feathers look dark and feel soft--pin-feathers they are called.

The blood dries up when the feather has unfolded to its full size, leaving it light and dry, with a horny part at the root that sticks in the hole where it grew, and a spray-like part that makes up most of the feather.

The horny part becomes hollow or contains only a little dry pith; when it is large enough, as in the case of a rowing feather from a Goose's wing, it makes a quill pen to write with.