Introduction

The top four google hits for “how many types of feathers birds have” indicate a general lack of agreement with Bird Watching Daily having 6, Cornell Bird Academy 7, Study 6, and Bird Watching Academy 5. All of these numbers are created by subjective grouping and are overly simplistic. A more accurate label for these sites would be feather categories. The approach in this article is a simplistic one, do feathers look or have characteristics that differentiate them from others and are those feathers found on a significant population. Not all bird species have bristles and others don’t have flight feathers. Evolutionary adaptations within tail feathers like twelve-wired bird-of-paradise vs the stiff rectrix of a woodpecker or the serrated leading edge of an owl primary vs an undifferentiated feather on a penguin’s wing are not separately categorized.

A chicken (gallus gallus domestic) was used as a baseline.

Parts of a Feather

To understand various types of feathers requires a knowledge of a feather’s structure and function. The basic parts of a feather are displayed in Figure 1.

A chicken tail feather displays the standard parts of a bird feather. The calamus (shaft or quill) is the base of the feather. It is hollow and without barbs. Extending from the calamus is the rachis (scapus). The base of the rachis is a section called the aftershaft that has downy barbs. The rachis ends with a section called the vane (or web) where barbs are attached at a 45 degree angle.

Barbs attached to the rachis come in three basic feather branching structures, pennaceous, plumulaceous and filoplume (discussed as a feather type below).

This chicken contour feather shows two types of feather structure. The pennaceous end is characterized by sturdy interlocking barbs that hold up to wind and water. The plumulaceous end looks loose and chaotic allowing trapped air to insulate the body.

Focusing on the pennaceous structure, the following figure shows barbs of a chicken tail feather attached to the rachis with increasing magnification. Well structured, very tight and very sturdy.

A closeup of some barbs follows. Each barb has a central ramus where barbules are attached at a 45 degree angle. Note the texture difference between the distal and proximal barbules.

Pennaceous microstructure consists of stiff rigid barbs that have a central ramus covered on two sides with barbules. Figure #5 is a close-up of a primary feather showing the barb details. The proximal and distal sides of adjoining barbs overlap in an X formation.

Barbules have hooklets (barbicels or hamuli) that act like Velcro (cocklebur origin) to interlock barbs together and stiffen the feather for flight. The woven surface also repels dirt and rain and protects against wind and sun. The figure above shows the hooklets on the pennaceous feather. There are always exceptions as in the case of cormorants whose feathers absorb water to help them dive and are why you often see their wings outstretched while sitting in the sun. If barbs get separated the bird reconnects them via preening.

Plumulaceous microstructure has barbs without hooklets as seen in the above figure. Figure 6 is various magnifications of the aftershaft of a contour feather. Note the barbs turn into a fluffy texture just a short distance from the rachis. The barbs end in a flexible ramus with longer and spaced barbules.

The barbs look in disarray as they don’t have hooklets to hold them together. This arrangement allows air to be trapped for insulation. The magnification below shows the barbules lacking the hooklets to hold them together.

Types of Feathers

Cornell lists seven basic feather types. This article identifies ten.

 

Flight feathers (remiges) are asymmetrical, with the leading edge being stiffer to prevent twisting as air flows over it. They come in three groups, primaries, secondaries and tertiaries. Most birds have 10 primaries that are counted from the inside out at the end of the wing. The primaries provide thrust. There are 10-14 secondaries in the middle of the wing and are counted from the outside in. The secondaries provide lift. The inner most flight feathers are tertiaries and do not perform an essential part in flight. Note that the wing feather does not have an aftershaft and downy barbs that would hinder flight. Melanin is the pigment that makes feathers black. Black feathers are more resistant to wear and tear, thus many birds have black outer primaries.

Flight primary feathers are attached directly to the bird’s small, fused “hand” bones. All the rest of the feathers are attached only to the skin. (Phillipsen 2020)

There are many flight feather modifications such as the owl whose flight feathers have a serrated front edge to maximize stealth in flight. At the other extreme, penguins have no flight feathers at all.

Tail feathers (rectrices) are variable with most birds having 12. Hummingbirds have 10 and other species range from 6 to 32. The feathers are arranged in a fan like structure and counted from inside out. The outer feathers are more asymmetrical than the inner ones. They have the same microstructure as the flight feathers and are used for steering.

There are many exceptions to the standard tail, the starkest of which is the twelve-wired bird-of-paradise. While processing feathers from a gila woodpecker I noticed some interesting morphology. The barbs at the end of the tail were very stiff and lacked barbules, very much like a plastic toothpick. About a third of the feather’s distal end had this feature. Going to the proximal end, the barbs gradually had more and more barbules till about half way down the feather, the barbs displayed normal pennaceous structures. The stiffness of the barbs supplants the need for hooklets to keep them in place. The barbs can move independently, much like a 3D contour gauge. These features assist a woodpecker with locomotion, hanging from a vertical or angled branch and stabilizes the woodpecker when hammering.

Contour feathers cover the entire body and are typically symmetrical. Some sites group primaries and tail feathers into a contour category. The aftershaft length is variable, with the shortest on the wing to minimize airflow disruption. The downy barbs are close to the body and the vanes create a shingle-like overlay that waterproofs the bird. The contour feathers that cover the base of flight feathers and above and below that base of the tail are called coverts. There are many names for various contour feathers depending on their location.

Here is a sample of the various names given to contour feathers. Starting top left clockwise, Red Junglefowl hackle feathers are on their necks, tail coverts on a Yellow-rumped Warbler, Turkey Vulture axillaries in the axilla (“armpit”), and lores in front of the eyes on a Yellow-eyed Vireo are just a few of the many contour feathers.

Semiplume feathers are a cross between downy and contour feathers. Unlike downy feathers, they have a long rachis with a long aftershaft section containing barbs with under developed hooklets. These feathers are disbursed under the contour feathers for insulation.

Body down (plumule) feather is the closest feather to the body and is not exposed to the elements. The barbs are without hooks allowing for a chaotic distribution creating the insulation. Down feathers have little to no rachis, are relatively short, and have spaced flexible barbs without hooklets. Feathers found in amber indicate that some dinosaurs were covered by them. The body down feathers have been used by humans for insulation in clothing and bedding. Feather gathering has been noted as far back as 200,000 years. Many birds use their body down feathers and those of others to line their nests.

Young birds are covered in natal down feathers (figure above left of tri-colored heron). Precocial nestlings from ducks and quail have them when born, altricial chicks develop natal down feathers within 6 days of hatching.

Powder down feathers (pulviplumes) are found on herons, pigeons and parrots. The tips of the barbules disintegrate creating a talcum like powder. These feathers never molt. A second kind of powder down creates the powder from cells that surround the barbules of growing feathers. The powder is used for preening and waterproofing.

Filoplume feathers, the third feather structure, have a short calamus and a long bare rachis that ends with a small tuft of barbs. Figure 15 shows a comparison to a human hair. Filoplumes come in varying lengths depending their location. Filoplumes are found under contour feathers with a heavy concentration around flight feathers. They are attached to sensory receptors in the skin that detect air pressure, wind and feather movement. Every bird has at least one filoplume per wing, tail or body feather. Devokaitis (2020)

While handling the flight feather I noticed a filoplume. On closer inspection there were 5. Since all the feathers were plucked, some skin was still attached to the calamus. Flight feathers have up to 12 filoplumes each. Devokaitis (2020)

There are no barbules or hooklets on this small tuft of barbs seen above. Some internet illustrations of filoplumes show barbules. After plucking a chicken, the residual filoplumes are so numerous it is recommended to singe them off.

In the hunt for ring-necked pheasant bristles I found some periorbital candidates. Looking like eyelashes,  closer inspection determined the feathers were actually undeveloped filoplumes. The figure above shows some tuft growth activity at the top of each feather. Bristles, if they have any barbs at all, would show at the base. At a quick glance, they do look like bristles.

Bristle feathers are found on many species. In the figure above going clockwise from top left Say’s Phoebe, Greater Roadrunner, Emu, Pyrrhuloxia, Plain Chachalaca, Great Horned Owl. As noted above, some immature filoplumes may look like bristles. Bristles are whisker like feathers typically found around the mouth, eyelids and nares. As can be seen in Figure 19, most bristles have a is a simple rachis that sometimes has a few barbs at the base. This is variable as some nightjars do have small barbs along the rachis.  Bristles assist in foraging, protection from airborne particles, sense airflow, assists obstacle avoidance, and are sensitive to touch and vibrations. Some sites state that bristles are used to funnel food. That was dispelled by an article about Willow Flycatcher bristles. (Conover 1980)

Auricular feathers are not on any feather type list. References for auriculars are cheek feathers and ear coverts and are found behind the eye. If the auricular is mentioned in an article, it is grouped in the contour feather category. There are some birds that don’t have auriculars as found in the following matrix.

Auriculars have a long stiff rachis. The barbs are spaced apart and have stiff rami. The vane is airy allowing for sound to pass. The ringed-neck pheasant auricular on the left is one inch long. Unlike body down barbs, auricular barbs hold their shape. The feather does not fit the pennaceous or plumulaceous feather structures.

Although the two examples above show a fanned out pattern (possibly due to static electricity), when on the bird the barbs sometimes compress, narrowing the amount of area covered. In the figure below, the Abert’s towhee on the left shows a fanned out feather while the ringed-neck pheasant (middle) and gila woodpecker shows that barbs closer together.

Below is a close up (50X 150X 300X) of a ring-necked pheasant auricular. Note the absence of barbules and associated hooklets. This design prevents dust from getting in the ear and muffles air turbulence while channeling sound into the ear.

Summary table of feather attributes

Periorbital feathers are intriguing and hopefully the subject of my next study. There is not enough information to determine if it has a unique feather structure.

Feather Facts

Feathers are made of keratin, the same material that makes up fingernails, hoofs, claws, hair, scales, beaks and talons.

Feather follicles are connected to each other by several muscles, notably by the smooth apterial muscle (counteracting horizontal movements of feathers), the smooth erector muscle (which lifts the feather up) and the depressor muscle (which pulls the feather down and counteracts the vertical rotation of feathers induced by airflows). (Delaunay 2020)

The cross section of a tail feather rachis changes from circular to rectangular towards the distal end. The ventral groove and ridges were found on the flight, tail, and contour feathers of the chicken. The void that you see in the figure below is natural. The pith is a stiff foam like material.

Feathers do not have nerve endings. Herbst corpuscles are pressure sensitive and the most widely distributed mechanoreceptors in birds. They are present at feather follicles and present in many other parts of the body. They resemble the Pacinian corpuscles of mammals and detect rapid mechanical deformation (vibration). Not every follicle has a corpuscle. (Van den Broeck, 2023) These corpuscles signal any movement in the feather.

Unlike human hair follicles, which are randomly distributed, feather follicles are arranged in a precise pattern so that feathers properly overlap. Follicles grow in patches called pterylae. There are parts of a bird that do not have follicles, but those bare spots are covered by their surrounding pterylae. On occasion a malformation of the follicle occurs resulting in two or more feathers being grown. This condition is called polyfolliculosis. (Rich 2023) The figure above shows two calami fused at the base and from a single follicle.

The preening (uropygial) gland produces oil used by birds to condition their feathers. This gland is on the bird’s back just above the base of the tail. The bird rubs its beak across the gland picking up the oil. It then applies it all over. The oil is like a hair conditioner that keeps the feathers healthy and aids in repelling water.

Sonations are sounds produced by feathers. Some male hummingbirds, like an Anna’s, use auditory displays as part of their mating ritual. The male hovers about 50 feet above the nest, then dives. At the last moment it pulls out of the dive causing a whistle sound made by the tail feathers.

Feathers get their color from three pigment groups – psittacin, carotenoid and melanin. Psittacin is only found in parrots, and carotenoids (yellow, orange, red, and greenish) are from diet and rarely occur. Melanin, from the Greek melanos – dark-colored, which comes in two forms: eumelanin and phaeomelanin. Eumelanin is seen as black, gray, or dark brown. Phaeomelanin creates warm reddish brown to pale buff. Melanin strengthens feathers. Birds like the snow goose, white ibis, and most gulls have black outer primaries or feather tips that take advantage of this trait.  So where do Blue Jays and Green Jays get their colors? Blue in birds is not a pigment; it is created through refraction like light passing through a prism. Green Jays combine blue refraction with a yellow carotenoid to create its green appearance. Hummingbird gorgets also use refraction to create their color and iridescence. Blues, violets, and white are all structural rather than pigments.

Licensed

Prior to starting the feather study I obtained a Scientific Activity License from the state of Arizona (good for one year) and can be renewed. The possession of feathers and other parts of native North American birds without a permit is prohibited by the Migratory Bird Treaty Act (MBTA 1918). There is no exemption for molted feathers or those taken from road – or window – killed birds.

References

Conover, Michael R.  and Miller, Don E. (1980) Rictal Bristle Function in Willow Flycatcher. The Condor,
Vol. 82, No. 4, pp. 469-471

Delaunay, Mariane G. (2020) Anatomy of avian rictal bristles in Caprimulgiformes reveals reduced tactile function in open-habitat, partially diurnal foraging species. Journal of Anatomy, 237:355–366. 

Devokaitis, Marc (2020) The Most Mysterious Feather: Filoplumes. Cornell  “https://www.allaboutbirds.org/news/the-most-mysterious-feather-filoplumes/#”

Phillipsen, Ivan. (2020) The Parts of a Feather and How Feathers Work. “https://www.scienceofbirds.com/blog/the-parts-of-a-feather-and-how-feathers-work”

Rich, Gregory, DVM. (2023) Polyfolliculosis in Birds. VCA Animal Hospitals “https://vcahospitals.com/know-your-pet/polyfolliculosis-in-birds”

Van den Broeck, Martine. (2023) Histology of Birds. Ghent University “http://www.histology-of-birds.com/galleries.php?id=74”

van Grouw, Hein. (2022) A Review of Nomenclature for Avian Color Aberrations. American Birding Association, North American Birds 73(1): 36-51, 2022

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