Feathers are a body covering unique to birds. They serve many purposes including flight, insulation, protection, water resistance, sensation, display and coloring. While no two bird species look the same, their feathers share some common characteristics. A typical feather has a hollow central shaft known as the rachis, which ends in a bare section called the calamus or quill. Branching off the rachis on either side are barbs. Each barb has small barbules extending from it, which serve to connect neighboring barbs together. When birds preen, one of the things they do is zip or hook their barbs together for weather resistance and aerodynamics.