barbule

And every quill and barbule of the feathers was painted on by hand.

"It's the size, shape, number, and pigmentation of nodes and their placement along the barbule that are key," he patiently explained.

He noticed that pressing the barbule would shift the color toward the blue, while swelling it with a chemical would shift it toward the red.

Plus; they aren't located throughout the barbule but are more distal, or located more toward the end of the barbule.

Depending on how closely the rods were spaced and how many layers of rods were stacked up, the barbule could appear anything from yellow to green to blue to brown.

As birds become flightless, their feathers soon lose the barbule hooks that keep them in shape, becoming more hair-like; when flight performance is of no significance anymore, this can improve the insulating properties of the plumage.