Epidermal Structures in Vertebrates
The outermost covering on the body of a vertebrate is the epidermis. Fish, amphibians, reptiles, birds, and mammals are all wrapped in this protective layer. The epidermis protects against scratches and abrasion, the elements, and microorganisms with bad intentions. It also forms a barrier that holds moisture inside the body.
New cells form deep in the Epidermis and are pushed outward as they age. Older epidermal cells in our skin form the stratum corneum. This means the “layer made of horn.”
Humans may not have horns growing out of our skins, but we do have hair and fingernails, which are made of the same stuff as cow horns, rhino horns, etc. And we get callouses, which are sort of little horn-like things projecting from our skin.
Other related epidermal structures in mammals include claws, hooves, and the baleen of whales.
Scales in reptiles, including those on bird legs and feet are also epidermal structures. Yes, I lumped birds in with the reptiles. Remember that birds are dinosaurs, and of course, dinosaurs are reptiles. But birds are still treated by most scientists as belonging to their own class, Aves. Maybe someday we’ll revise our classification systems, our taxonomies, to more correctly reflect the reptilian nature of birds. I should point out that fish scales are derived from the dermis—a deeper layer of the skin—not the epidermis. So they don’t share the same origin as these other structures we’re talking about.
Pterosaurs, those flying reptiles of the Mesozoic era, had some hair-like and feather-like things decorating their skins. It’s still uncertain whether those structures shared a common origin with bird feathers. For the record, Pterosaurs are definitely not the ancestors of birds. They aren’t even technically dinosaurs.
And finally, we come to bird beaks and feathers. These are yet more creative expressions of the epidermis in animals. Scientists have shown that feathers, hair, horns, etc. all grow from the same, identical embryonic tissue. They are sort of elaborations of the same theme. Feathers are definitely the most elaborate of these elaborations.
The raw material that natural selection has molded into all these jazzy skin adornments is keratin. Keratin is a protein that accumulates in the ol’ stratum corneum.
Keratin is a polymer. It shares some properties with man-made plastics, which are also polymers. Take a look at your fingernail, which is made of keratin.
There are two main forms of keratin in animals: alpha and beta. Alpha keratins are present in all vertebrates. This is the sort of keratin protein in hair and other other mammalian structures.
Beta keratins, however, are found in only reptiles and birds. They form a harder, tougher material than alpha keratins. Beta keratins make reptile claws and scales, the outer layer of turtle shells, bird beaks, and feathers.
What Are The Parts of a Feather?
Okay, now let’s take a look at the structure—the architecture—of a feather. Luckily, you already know the blueprint of a typical feather. Let’s see if we can add some detail to your existing mental image.
For starters, we have a central shaft that forms the main axis of the feather. The shaft is stiff but also elastic: it bends when force is applied but springs back to its original shape when the force is removed.
The relatively thick base of the shaft is hollow. This part is called the calamus and it anchors the feather in the skin of a living bird. The rest of the central shaft is the rachis. Unlike the hollow calamus, the rachis is solid. It tapers to a fine point at the tip of the feather.
Spreading out from both sides of the shaft are the vanes. These are broad, flat structures that form a single two-dimensional plane. The vanes appear to be sort of solid, but you know from experience that they’re actually made up of many, tiny, hair-like structures, running in parallel. We call these barbs. They branch off of the shaft and are tightly packed together, side by side.
Now let’s zoom in to look at the microscopic structure of the barbs. We’re getting into the territory of stuff you can’t see by just holding a feather and staring at it.
We see the barbs running parallel to each other, row after row. But now we see that each barb has its own little branches coming off two sides. These are the barbules. Barbs branch off the central shaft and barbules branch off the barbs.
Here’s where it gets really interesting. Zooming in further you can see that each tiny barbule has teensy weensy hooks coming off of them at intervals. These hooks—called barbicels or hooklets—grab onto the next barbule over, the one closer to the feather tip. The hooklets function to keep the barbs zipped together nice and tight. In fact, this interlocking mechanism really acts like a zipper. Grappling hooks also come to mind.
Tug on the vane and you can see how the barbs cling together. But then, under enough strain, they suddenly “unzip.” The barbs can be smoothed back together between fingertips, letting the hooklets work their magic and the gap is sealed up, and the vane is whole again.
This is one of the main things birds are doing when they preen. They use their bills or feet to zip together gaps between the barbules of their feathers.
So that’s how a typical feather is put together. You’ve got a central shaft which is divided into the calamus and the rachis. Barbs are branches coming off the shaft. They form the vanes. Each barb has its own central axis with barbules branching off on two sides. And the barbules have hooklets.
Crawling around in the branches of this feather forest are some spooky ectoparasites: feather lice, louse flies, and feather mites. Birds have to deal with these creepy crawlies to keep their feathers in good shape. I’ll go into ectoparasites at some point in the future.
Feather types can be divided into two broad categories: Pennaceous and Plumulaceous. So far I’ve been describing the classic pennaceous feather.
Although plumulaceous feathers have most of the same components, they don’t have distinct vanes. They’re fluffy and mostly shapeless. This is because the barbs and especially the barbules are long and very flexible. And, importantly, the barbules don’t have hooklets. Down feathers are plumulaceous. We’ll get to down and the other types of feathers in just a moment.
Follicles and Pterylae
Before we do that, I want to mention follicles. A follicle is the special structure on a bird’s skin that produces a feather. These are the bumps you’ve seen on a raw, plucked chicken. Specialized cells in the follicle produce the keratin that fuses together into a feather.
Unlike hair follicles in humans, follicles on birds are arranged neatly and regularly across the skin. Feathers, in general, need to be in precise locations on a bird's body, so that they overlap just so and serve their various functions effectively.
Follicles are grouped into distinct patches called pterylae. Between these pterylae are bare patches without follicles and therefore with no feathers. You rarely see these bare patches on a living bird because feathers from the neighboring pterylae cover them up.
Depending on the type of bird and its size, it might have a total of only 1,000 feathers on its body or it might have up to 25,000. Hummingbirds are on the low end of this spectrum; swans are on the high end.
What Types of Feathers Do Birds Have?
Contour feathers are most of what you see on the surface of a bird’s body. They give a bird its characteristic shape, it’s contours. The silhouettes of naked chicken and a fully feathered one are very different, as I think you can picture. It’s contour feathers that fill in the empty spaces to round out and streamline the bird’s shape.
A typical contour feather has a pennaceous part, with vanes and all the substructures. It may also have a portion near the base that is plumulaceous. The pennaceous portion—the outer portion—of these feathers overlap like fish scales or roof shingles.
Tiny muscles around the follicle keep a contour feather in its proper orientation. These muscles also allow for some control of feather position. Contour feathers can be raised or lowered voluntarily—usually as whole tracts of feathers, not individually. Birds frequently fluff up or compress their plumage to regulate their body temperature. But there are other reasons they move their feathers. For example, a bird might raise its contour feathers to make itself look bigger and more menacing to threaten a rival or a predator.
Flight feathers are specialized contour feathers of the wing and tail. They’re usually the largest and stiffest feathers on a bird, and they’re almost entirely pennaceous in structure.
The size, stiffness, and shapes of flight feathers all contribute to help a bird overcome gravity and take to the air.
Flight feathers of the wing are called remiges. The remiges of the outer wing are called the primaries, and those of the inner wing are the secondaries. Each of these flight feathers has asymmetrical vanes. The vane on the leading edge, facing into the wind, is narrower than the vane on the trailing edge. This allows the feather to withstand the force of rushing air during flight.
The tail feathers are called rectrices. They usually have symmetrical vanes. They form the lovely fan shape of the tail. The two central rectrices are anchored to the tailbone, which is called the pygostyle.
Flight feathers on the wing, the remiges, are connected directly to a bird’s arm bones. This differs from other feathers, which are anchored in the skin only. The attachment points of the secondaries to the ulna bone are called quill knobs. They’re visible as little bumps on the bone.
Down feathers are hidden beneath a bird’s coat of contour feathers, at least in adults. A down feather is entirely plumulaceous. It has either a very short shaft or none at all.
There are several types of down. Natal down is what many baby birds are born with. It’s most obvious in precocial chicks, the ones that are ready to rip as soon as they’re born, like chickens and ducks. You can picture the soft fuzz that covers these little dudes. As these chicks age, contour feathers grow out of the same follicles that produced the natal down.
Body down is the typical down of adult birds. A body down feather grows from a specialized follicle that makes only this type of feather.
Then we have powder down. These are pretty interesting feathers. A powder down feather grows continuously throughout a bird’s life and is never molted. It breaks apart at the tip, disintegrating into a fine dust of keratin particles. This powdery substance works its way into the entire plumage of a bird. Although ornithologists don’t know the function of powder down, it’s likely involved in repelling water or maybe defense against parasites.
Only birds of certain families have powder down feathers. These include pigeons, parrots, herons, and a few others.
Semiplume feathers are something halfway between a down feather and a contour feather. The barbules on a semiplume feather lack hooklets. So semiplumes have less structure than contour feathers, but they aren’t quite as amorphous as down.
These are specialized feathers that look to us more like stiff hairs. Most bristles lack barbs; some have a few barbs near the base. We see bristles mostly on the heads of birds.
Some good examples are the glamorous eyelashes of some hornbills and the Secretarybird in Africa. Also, members of the nightjar family Caprimulgidae have well-developed bristle feathers surrounding their beaks.
And our final feather type is the filoplume. These, too, are hairlike, but they’re more slender. Some filoplumes have a tiny tuft of barbs at the tip. Filoplumes are scattered around the plumage, hidden among the contour feathers. They connect to nerves in the skin and are thought to have a sensory function. Filoplumes allow a bird to detect air movements and also movement within their plumage.
Functions of Feathers
Feathers are critically important for flight in birds. The flight feathers form long, broad surfaces that have the physical properties needed to generate lift and thrust. They’re lightweight and stiff, but also flexible in just the right way.
Contour feathers of the arm are also super important in giving the wing its airfoil shape, like the wing of an airplane. Contour feathers smooth a bird's shape to make it more aerodynamic overall.
Remember that the rectrices, the tail feathers, are also flight feathers. The tail itself has a shape that provides lift and maneuverability in flight.
Birds are endotherms like us. In other words, they’re warm-blooded. So, like us, they need to maintain their body temperatures within a narrow range. Most birds have a body temperature around 106℉ (41℃).
Feathers provide the insulation a bird needs to minimize heat loss in the cold. Down is famous for its outstanding performance as insulation. Other feather types also contribute to temperature regulation, too. Contour feathers have their fluffy, plumulaceous bases and they shield a bird’s skin from solar radiation. Semiplumes also provide some insulation.
The woolly structure of down is built up from a tangle of long, soft barbules. Within this matrix, at the microscopic level, air moves slowly. It gets caught up in all those barbules. So air warmed by a bird’s hot little body stays trapped in the down feathers, close to the skin.
This is how penguins can be comfortable in the frigid waters of Antarctica, and how migrating Bar-headed Geese can fly at 29,000 feet. As far as I know, human engineers still haven’t come up with a synthetic insulation that’s as lightweight, compressible, and efficient as bird down. I bet they’ll figure it out at some point.
Until then, there’s still a big market for bird-down for insulation. There’s a problem with this that I should mention. Some down is plucked from live birds, which is cruel and horrific. This sort of harvest is illegal in the US, Canada, and most of Europe. But it’s legal in many other countries. Some sources say that only 1% of the world’s down supply comes from live-plucked birds. Other sources say it’s up to 80%. Some companies like IKEA and Patagonia now guarantee that their down does not come from sources that live-pluck birds.
It’s easy to argue that just about any source of down—live-plucked or not—involves some inherent cruelty. So, my bird-loving friend, I invite you to give this some thought before buying a down-filled product.
But there is one source of down that’s delightfully free of cruelty. Common Eiders are beautiful, seafaring ducks of the North Atlantic. Living in that environment, you can imagine they have some pretty hard-core down.
Female Eiders pluck some of their down feathers to line their nests, which is brilliant. They aren’t unique in this behavior, since many other water birds also use down in their nests. But eiderdown is extra warm and soft.
For centuries, people in Iceland have been harvesting this Eider down from nests. But they take the down only after the nests have been abandoned by the birds. The ducks show up year after year to some farms, waddling out of the sea to build their nests. There’s a kind of symbiotic relationship between eiders and their farmers: the humans provide shelter and protection, the ducks provide the fluffy goods. Harvesting and processing Eider down is quite a process. So it should come as no surprise that it’s super expensive. An eiderdown duvet can cost thousands of US dollars.
Feathers provide most birds with a water-resistant—if not water-proof—outer covering. If down acts like a puffy insulating parka, the contour feathers are like the rain jacket.
The microscopic structure of the barbs and barbules in contour feathers gives them their water repellent property (watch). Tiny air spaces between barbs cause water to form spherical beads that just roll off. Like water off a duck's back, right?
Birds condition their feathers with oil produced by the preen gland, also called the uropygial gland. This gland sits just above the base of a bird's tail feathers, on its back. A bird runs its bill across the gland to collect some oil, then rubs the oil all over its plumage. The oil is kind of like a hair- conditioner for feathers. It keeps them in good working order and helps maintain their water repellency.
One of the things we love most about bird feathers is how colorful they are. The plumages of countless species are jaw-droppingly gorgeous. They combine colors in ways that even a skilled artist might think would never work together.
To some extent, it’s just a quirk of biology and evolution that we humans think birds are pretty. We just like bright colors. Colorful birds aren’t meant to make us happy, even though they do. As you know, the vivid colors of bird feathers function as a display, to other birds, mostly.
Distinct plumage patterns—colorful or not—are useful to birds in species recognition. It’s not uncommon for several bird species in a habitat to look pretty similar. It’s important for a bird to know which of its neighbors are members of the same species. There are a couple reasons for this.
One is that you don’t want to waste your precious time and energy courting a member of another species, trying to win their affection. They’ll probably just blow you off and you’ll be left out in the cold. And if you just happen to be successful in mating with another species…? Well, your hybrid offspring resulting from the mating are likely to kind of suck. They’ll have relatively low fitness in terms of natural selection. This is one explanation for plumage differences in closely related bird species that share a habitat.
It’s also good to know who’s a member of your own species because these guys are likely to be your primary competitors for limited resources. They eat the same food as you, they want the same high-quality mates, and they use the same nesting sites. You need to keep your eye on them. Another species with similar, but distinct plumage probably uses different resources, so you can maybe just ignore them.
Here’s an example. There are two chickadee species that I see in my backyard and nearby woodlands: The Black-capped Chickadee and the Chestnut-backed Chickadee. This is in Oregon, in the US.
Superficially, these species look very similar. They have white cheeks, black caps and throats, and gray bodies and wings. But with even a quick observation, we can see that the Chestnut-backed Chickadee is aptly named. It has a rusty red patch of feathers that wraps around like an adorable little vest. That’s the most obvious difference, from our human perspective.
Even though these birds flit around in the same general area and do cross paths, they forage in different types of trees and have different nesting habitats.
When a Black-capped Chickadee sees a chickadee-shaped bird on a nearby branch, it can safely ignore the other bird once it sees that little red vest.
The most widely understood use of feathers as a display is for mate attraction. We’re all familiar with the amazing displays of male peacocks and birds of paradise. So much of the bright coloration we see among male birds is the result of female choice in the process of sexual selection.
Different types of feathers have been co-opted for the purpose of mate attraction through sexual selection. Contour feathers that provide a bird’s protective outer coat have, in many species, evolved into elaborate and/or colorful structures used for display.
For example, the Ruff is a large shorebird, a member of the sandpiper family. It breeds across northern Eurasia. Breeding males have elongated, conspicuous contour feathers on their necks. They look like they’re wearing a ruff… you know those silly, white collars that Shakespeare and his buddies wore in Elizabethan times? Male Ruffs—the birds—use their extraordinary neck feathers to impress females.
Flight feathers of the wing are so important for their primary function that they aren’t as frequently modified by sexual selection into weird structures. But they can certainly be colorful for the purpose of mate attraction.
Tail feathers, on the other hand… oh boy are there so many ways these have been shaped into amazing displays for attracting mates. Turkeys, lyrebirds, widowbirds, hummingbirds, paradise-kingfishers, paradise-flycatchers, the Scissor-tailed Flycatcher… There's a long list of species whose males have long, bedazzled tail feathers.
Visual vs Auditory
The displays we’ve been talking about are visual displays. In general, birds have great eyesight. Read my article about vision in birds if you’d like to learn more about that.
Birds can see more colors than we can, including some in the ultraviolet end of the spectrum. It seems many males have patches of feathers that shine brightly in UV light, and this is attractive to females.
Some birds also use feathers in auditory displays for mate attraction. Males of some hummingbird species, for example, make display dives directed at a perched female spectator. Air moving fast over the male’s tail feathers produces a loud whistling sound at the end of the dive. This is intended to accentuate the impressiveness of the overall display. Sounds produced by feathers this way are called sonations.
The Club-winged Manakin (Machaeropterus deliciosus) has an amazing display that involves feather sonation. This small, egg-shaped bird lives in mossy Andean forests of western Ecuador and Colombia. The males have a scarlet cap, brownish-red bodies, and black wings.
Males display for females by raising their wings over their backs and vibrating them rapidly. The wings vibrate 107 times every second, which is even faster than a hummingbird flaps its wings in flight.
And it gets even cooler: several of the Club-winged Manakin’s secondary flight feathers, on the inner wing, have uniquely thickened shafts. While the wing is being vibrated, these club-shaped feathers knock against each other to produce a loud ting sound.
More than 20 other manakin species make noises with their wings in courtship displays. The Club-winged Manakin has just taken this to a fantastic extreme.
Being colorful and conspicuous has its advantages, as we’ve discussed. But many birds use feathers to achieve the opposite effect. They use them for camouflage.
Birds are tasty little bundles of meat that are targeted by many predators. Ground-nesting birds or those that just spend a lot of time on the ground are particularly vulnerable. So birds like grouse have plumages that provide remarkable camouflage. Females of many types of birds tend to be less conspicuous and more camouflaged. This makes sense for species where the female is the one who spends most of the time at the nest.
Examples of feather patterns and coloration providing excellent camouflage are numerous. Some of the most impressive are found among several families of nocturnal birds. The frogmouths of southeast Asia and Australia, the potoos of the neotropics, and many owls use their feathers to mimic tree branches or tree bark. These birds are active at night and rest on perches during the day. To blend in, they simply sit still, close their enormous eyes, and assume a posture that allows them to look just like a chunk of wood.
Birds in these three families are not closely related. Similarities in the appearances and behaviors of frogmouths, potoos, and owls are largely the result of convergent evolution. They share a similar need to be camouflaged while sleeping on their daytime roosts. Natural selection has endowed all of them with feathers that look convincingly like tree bark.
So these are the main uses of feathers: flight, insulation, display, and camouflage. There are a few other, more obscure ways that birds use feathers, but we’ll leave those for another day.