Quills are the stiff, hollow hairs that make up the feathers of birds. They are primarily used for flight, providing the necessary structure and aerodynamics. Quills can also be found in some mammals, such as porcupines, where they serve as a defense mechanism against predators. Historically, quills were used as writing instruments before the invention of modern pens. Writers would dip the pointed end of a quill into ink to create letters and documents. This method was popular in the Middle Ages and is still appreciated today in calligraphy and art for its unique line quality.