What is a contour feather?

Contour feathers cover most of the body of a bird. They provide streamlined protection to help birds fly. Contour feathers have stiff barbules but unlike flight feathers, they're not interlocked; they're free to move against one another. This flexibility lets birds move their wings and stay warm. The long, thick rachises of the contour feathers provide rigidity and strength. When a bird is gliding, the vanes of the wings spread widely from their main axis, maximizing surface area and lift, while minimizing friction.