Fiery-throated Hummingbird
Unique to birds and their dinosaur ancestors, feathers have evolved into impressive biological structures that come in a surprising diversity of colors and forms.
As feathers grow, they mature into highly branched structures. Careful study of this process inspired new hypotheses about the evolution of feathers through stages of increasing complexity. Newly unearthed dinosaur fossils from China and Canada have supported these hypotheses by providing specimens from each stage in the proposed evolutionary history—a clear example of how investigating biological structures across contexts can create scientific breakthroughs.
Thorough understanding of biological structures like feathers requires examination from many angles. We now recognize that how feathers function is intricately connected to how they’re structured, and how they grow is closely linked to how they evolved. Read excerpts from an article from the Cornell Lab of Ornithology’s Bird Lab. For more about the structure, function and evolution of feathers read the full article All About Feathers,
FEATHER STRUCTURE
Although feathers come in an incredible diversity of forms, they are all composed of the protein beta-keratin and made up of the same basic parts, arranged in a branching structure. In the most complex feathers, the calamus extends into a central rachis which branches into barbs, and then into barbules with small hooks that interlock with nearby barbules. The diversity in feathers comes from the evolution of small modifications in this basic branching structure to serve different functions.
Downy feathers look fluffy because they have a loosely arranged plumulaceous microstructure with flexible barbs and relatively long barbules that trap air close to the bird’s warm body.
Pennaceous feathers are stiff and mostly flat, a big difference that comes from a small alteration in structure; microscopic hooks on the barbules that interlock to form a wind and waterproof barrier that allows birds to fly and stay dry. Many feathers have both fluffy plumulaceous regions and more structured pennaceous regions.
Feathers fall into one of seven broad categories based on their structure and location on the bird’s body.
Wing feathers
The wing feathers specialized for flight are characterized by uniform windproof surfaces, or vanes, on either side of the central shaft that are created by an interlocking microstructure. Also called remiges, these feathers are asymmetric with a shorter, less flexible leading edge that prevents midair twisting.
Tail feathers
Most tail feathers, or rectrices, feature an interlocking microstructure similar to wing feathers. Arranged in a fan shape, these feathers support precision steering in flight. Typically, birds have six pairs of feathers on the tail, which display increasing levels of asymmetry toward the outer pairs. In some birds, tail feathers have evolved into showy ornaments that are useless in flight.
Contour feathers
Contour feathers are what you see covering the bird’s body and streamlining its shape. Arranged in an overlapping pattern like shingles, the waterproof tips are exposed to the elements and the fluffy bases are tucked close to the body. Sometimes brilliantly colored or uniformly drab, contour feathers can also help the bird show off or stay camouflaged. Contour feathers on the wing, called coverts, shape it into an efficient airfoil by smoothing over the region where the flight feathers attach to the bone.
Semiplume
Mostly hidden beneath other feathers on the body, semiplumes have a developed central rachis but no hooks on the barbules, creating a fluffy insulating structure.
Down
Similar to semiplumes with an even looser branching structure but little or no central rachis, down feathers are relatively short and positioned closest to the body where they trap body heat.
Filoplume
Short simple feathers with few barbs, filoplumes function like mammal whiskers to sense the position of the contour feathers.
Bristle
Bristles are the simplest feathers, with a stiff rachis that usually lacks barb branches. Most commonly found on the head, bristles may protect the bird’s eyes and face.
Feather Function: What do feathers do?
Each feather on a bird’s body is a finely tuned structure that serves an important role in the bird’s activities. Feathers allow birds to fly, but they also help them show off, blend in, stay warm, and keep dry. Some feathers evolved as specialized airfoil for efficient flight. Others have been shaped into extreme ornamental forms that create impressive displays but may even hinder mobility. Often we can readily tell how a feather functions, but sometimes the role of a feather is mysterious and we need a scientific study to fill in the picture. FLIGHT
Flight
The primary and secondary wing feathers, or remiges, permit birds to take to the skies. Unlike other feathers, remiges are anchored to bone with strong ligaments so they can withstand the demands of flight and be precisely positioned. The primaries are longest of the flight feathers. They occupy the outer half of the wing, can be controlled and rotated like rigid fingers, and provide most of the bird’s forward thrust. While secondaries cannot be controlled as extensively, they provide most of the lift by overlapping to form an efficient airfoil. Tail feathers, or rectrices, are also classified as flight feathers. They are essential for steering, but only the two most central feathers attach to bone.
Display
Breeding adult male. Photo: Harry Collins/Audubon Photography Awards
Some feathers are so highly modified for display that they almost don’t look like feathers at all. For example, the iridescent spiral from a King Bird-of-Paradise (Cicinnurus regius) tail functions as an ornament in the male’s courtship display. Structurally, the feather is bizarre, with a bare rachis that ends in a tight spiral of barbs and barbules arranged only on one side of the rachis to form an eye-catching brilliant medallion. Read excerpts from the Cornell Lab of Ornithology’s Bird Lab. To take a deeper dive into the stucture, function, and evolution of feathers go to All about Feathers.
Modified contour feathers on the head are also commonly used in courtship displays. For example, the male Wood Duck’s (Aix sponsa) crest forms a colorful fan that completely changes its head shape. During this transformation, the bird elevates thousands of tiny feathers in unison by manipulating muscles just under the skin.
Not all fancy feathers are used to woo a partner; some are used in displays of aggression. For example, Blue Jays (Cyanocitta cristata) keep their crests lowered when they are at rest or with family and flock members, but raise them during aggressive interactions.
Insulation
Have you ever wondered why some birds hatch naked while others are covered in a coat of fuzzy feathers? Many young water birds must be able to swim and forage alongside their parents almost immediately after hatching. These precocial chicks hatch with a full coat of natal down to keep them warm in cold water. Young Mute Swans (Cygnus olor) for example, hatch with a fuzzy coat of natal down and after a few weeks, replace the natal down with an inner layer of adult down and an outer coat of contour feathers. In contrast, the young of many songbirds are born completely naked.
These altricial species stay warm by absorbing heat from attending parents and huddling together in an insulated nest. Utterly dependent at hatch, altricial species, like Purple Martins (Progne subis), require lots of parental care.
Waterproofing
Arranged in an overlapping pattern on a bird’s body to expose the waterproof tips, contour feathers allow water to roll right off a bird’s back. Birds constantly maintain their waterproof coat through extensive grooming, or preening to ensure that every feather is in good shape. The interlocking structure is so important that any disruption to it—such as if spilled oil coats the feathers—leaves the bird waterlogged and helpless. For ducks and birds like the Common Loon (Gavia immer) that spend most of their time in the water, maintaining a waterproof coat is critical for survival.