Feather

Feathers are complex epidermal growths that form a distinctive outer covering called plumage. This covering appears on all birds and once appeared on some non-avian dinosaurs and other archosaurs.

Feathers are considered a complex evolutionary novelty among vertebrates. They serve many vital roles for survival. They aid in flight and provide thermal insulation to keep birds warm. They also help with waterproofing the body.

Additionally, feather coloration is essential for communication and protection through camouflage.

Feathers grow from tiny follicles located in the epidermis, which is the outer skin layer. These follicles produce specific proteins known as $\beta$-keratins.

These protein strands are hydrogen-bonded into $\beta$-pleated sheets. They are further twisted and crosslinked by disulfide bridges. This chemical structure makes feathers tougher than the $\alpha$-keratins found in mammalian hair or hooves. While scientists do not know the exact signals that trigger growth, they have found that a transcription factor called cDermo-1 induces feather growth on the skin.

There are several distinct types of feathers categorized by their structure and location. Vaned feathers, also called pennaceous or contour feathers, cover the exterior of the body.

Within these, the remiges are the flight feathers of the wing, while the rectrices are the flight feathers of the tail. Down feathers grow underneath these vaned feathers. They lack barbicels, which are tiny hooks, so their barbules float free to trap air for insulation.

A third, rarer type is the filoplume. These are hair-like structures often hidden near the skin.

Some birds also possess rictal bristles, which are stiff feathers found around the eyes and bill.

A typical vaned feather has a specific mechanical design. It features a central shaft called a rachis. Fused to this rachis is a series of branches known as barbs. These barbs further branch into smaller structures called barbules. To keep the feather together, the barbules use minute hooks called barbicels for cross-attachment.

At the base, the rachis expands into a hollow, tubular structure called the calamus, or quill. This quill inserts directly into the skin follicle. In some species, hatchlings possess special natal down feathers called neossoptiles before their adult feathers emerge.

Feather color is produced through pigments or microscopic structures. Most pigments are melanins, which create browns, blacks, and greys. Carotenoids produce reds, yellows, and oranges.

Some birds, like turacos, have unique pigments called turacin and turacoverdin. Other colors come from structural coloration. This happens when microscopic structures refract, reflect, or scatter light. This process creates blue colors and iridescence.

In some cases, colors may be altered by secretions from the uropygial gland, also known as the preen gland. This gland also provides oil that may inhibit feather bacteria.

Humans have used feathers for many practical and cultural purposes throughout history. Large feathers from geese were historically used to create quill pens. In fact, the word "pen" is derived from the Latin "penna," meaning feather.

Feathers are also used in high-class bedding like pillows and mattresses because they trap heat. Down from geese and eider ducks has great loft, meaning it can expand to trap insulating air.

Today, many feathers used in fashion come as a waste product from poultry farming. This includes feathers from chickens, turkeys, and ostriches.

Feathers are deeply connected to the biology and environment of the bird. For example, cormorants have feathers that soak up water to reduce buoyancy, helping them swim submerged. Some birds, like herons, use powder down feathers to help waterproof their plumage.

Color patterns are also vital for social systems. Striking color differences are often part of sexual dimorphism, helping birds select mating pairs. Even the way feathers are distributed on the skin, known as pterylosis, helps scientists understand evolutionary relationships between different bird families.