Mar 112024
 

Auriculars and the Sonus Feather

A detailed study of bird auriculars

Species count: 45
Orders: 14
Plucked feathers: 3094

Feather Series

site under construction

 

Auricular Definition

Auricular feathers cover the bird’s ear which is inferior and posterior to the eye. These feathers are also called ear coverts or an ear patch. Currently auriculars are a subset of contour feathers. Auriculars are not usually noticeable but occasionally a bird will raise them.

Inca Dove auriculars

Inca Dove auriculars

White-winged Dove auriculars

White-winged Dove auriculars

Types of Auricular Feathers

To date the research has found four types of auricular feathers. All birds in the study group except the Ostrich, have sonus feathers (author’s term) of various morphologies. The Ostrich has bristle feathers for ear coverts and the American Kestrel has some bristles amongst its sonus feathers. The Costa’s Hummingbird has some contour feathers covering its meatus. Finally, all the sources agree that there are filoplumes associated with all feathers. The sonus is no exception. 

From left to right, an Ostrich bristle feather, a Barn Owl sonus feather, a Costa’s Hummingbird contour feather, and the far right, a Peregrine Falcon meatus showing filoplumes.

auricular feather types

auricular feather types

Meatus

The meatus is the external opening to the ear canal. It is situated slightly behind and below the eye. All of the birds in the study, with the exception of the Barn Owl, have auricular feathers covering the meatus. The Barn Owl has a pre-aural flap, called an operculum, covering the meatus which itself is covered by sonus feathers. The meatus of the Ostrich (OSTR) and the Ring-necked Pheasant (RNEP) are covered by different types of auricular feathers. In both cases, although not discernible in the picture for the RNEP, the meatus can be seen through the auriculars. 

Ostrich bristle auriculars and meatus

Ostrich bristle auriculars and meatus

Notice in the RNEP series that the sonus feather is fully splayed in the first frame. This is due to static electricity. When covering the meatus they lay compact resulting in greater coverage. Once the sonus is plucked, measuring the width would not be a valid measurement.

Ring-necked Pheasant sonus auricular and meatus

Ring-necked Pheasant sonus auricular and meatus

The Barn Owl (BANO) has the only bare meatus in the study group. It is bordered by the ruff, operculum and specialized sonus feathers. The Greater Roadrunner (GRRO) has the largest meatus in relation to its skull size in the study group. 

Barn Owl and Greater Roadrunner meatus

Barn Owl and Greater Roadrunner meatus

The Dark-eyed Junco (DEJU) and Abert’s Towhee exemplify the most common look of the meatus and the surrounding auriculars, sonus in this case.

Dark-eyed Junco and Abert's Towhee meatus

Dark-eyed Junco and Abert’s Towhee meatus

Meatus Scaling

Measurements were made of the meatus to calculate its area and the skull to estimate volume (see methods). A ratio was calculated and plotted. When plotted, the ratios for the Great Horned Owl and the Ostrich caused a loss of fidelity for data on the x-axis. The pictures on the right of the graph estimate where the ratios would plot to the y-axis. 

Meatus Area vs Skull Size

Meatus Area vs Skull Volume

As the skull reduces in size (left end of X-axis) the meatus becomes relatively larger (top end of Y-axis). This indicates that there is a limit to reduction in meatus area as the skull gets smaller. The meatus of a hummingbird is 6 times larger than an ostrich on a relative scale. 

An interesting article about estimating t-rex brain size introduced me to the concept of scaling.13 As seen in the above graph, it is difficult to compare a hummingbird to an Ostrich. Comparing an elephant to a mouse would make the x-axis kilometers long.14 To allow for comparisons of Ostriches to hummingbirds a log-transformation chart is used. The same data as the graph above is used for this next graph. 

Meatus Area vs Skull Size

Meatus Area vs Skull Volume

In this graph the orange points are hummingbirds which visibly seem to fit with the Ostrich plotted on the upper right. The data interpretation says that as skulls get larger the meatus area gets proportionately smaller. 

The yellow diamonds belong to the five ducks in the study. In comparison to other species with the same skull size, the ducks have a smaller than expected meatus area. This would fit if a smaller meatus helps protect the duck’s ear from water. The two lowest duck points on the graph are diving ducks, further supporting the evolutionary pressure to protect the ear.
The data point below and to the right of the ducks belongs to the chicken. Domestic chickens are raised in tight quarters without predators. Over 70 billion chickens are raised each year compressing evolutionary changes. A smaller meatus would seem a likely outcome.

Sonus Feather Morphotypes

Sonus Afterfeather

Afterfeathers (hypopnea) are a secondary feather growing from the dorsal side of a contour feather. It emerges at the base of the vane. The barbs are plumulaceous that sometimes attach to a rachis as in this case. Their appearance is like a downy feather and serves the same purpose. The afterfeather’s rachis was bent away from the contour feather for this photograph. 

Peregrine Falcon breast contour feather with afterfeather

Peregrine Falcon breast contour feather with afterfeather

Modified afterfeathers are found on some sonus feathers. They do not have the downy attributes of a contour afterfeather but have the same emergence location. The microscopy of a sonus afterfeather pulled from a Peregrine Falcon (PEFA) shows a sonus type structure. The first frame shows the afterfeather rachis emerging from the sonus feather at its proximal end. The other three frames show the proximal, medial, and distal portions of the afterfeather. Note the lack of barbs.

sonus afterfeather

sonus afterfeather

Four orders displayed this morphotype across all the species studied within that order. Piciformes’ Acorn Woodpecker (ACWO) and Gila Woodpecker (GIWO). Falconiformes’ American Kestrel (AMKE) and Peregrine Falcon PEFA. Accipitriformes’ Cooper’s Hawk (COHA), Harris’s Hawk (HASH), Sharp-shinned Hawk (SSHA) and Red-tailed Hawk (RTHA). Anseriformes’ American Wigeon (AMWI), Common Goldeneye (COGO), Green-winged Teal (GWTE), Lesser Scaup (LESC), and Norther Shoveler (NSHO). Conclusions on function and prevalence across these orders have not yet been made. 

sonus afterfeathers

sonus afterfeathers

Questioning Auricular Attributes

Ear protection, wind dampening, reflecting or directing sound are all attributes given to auricular feathers. In Lederer’s study of bristle feathers he observed “attributes seem to be casually accepted without definitive evidence.”9

Dust Protector

It is puzzling to read that primary purpose of Barn Owl (BANO) auriculars is to protect the ruff from dust.12 The immediate reaction is to question the environment of raptors as being dusty. Two birds that thrive in a dusty environment are the Curve-billed Thrasher (CBTH) and Rufous Hummingbird (RUHU). The CBTH forages for food by sweeping the ground turning over rocks and litter and sometimes pecks into the dirt like a woodpecker. The RUHU harvests bugs and nectar from flowering plants subjecting its face to pollen and dust. A hummingbird will feed over a thousand times per day.

These are the sonus auriculars from the three species.

sonus feathers

sonus feathers

 

The BANO’s feather is open with no discernible barbules while the two species from a dusty environment show growth along the rachis and developed barbules. A close up look follows.

distal microscopy of sonus feather

distal microscopy of sonus feather

 

It is clear that the sonus feathers for the CBTH and RUHU are adapted to account for the dust environment. If the primary function of the BANO sonus feather is dust protection, why not have similar adaptations shown in the other two species. More puzzling is why the sonus feather completely covers the area inside the ruff. A sub optimized dust protector on the cheek of a bird would be better served by an ordinary contour feather if protection is its primary function.
Review of sonus morphotypes indicates there are adaptations to a dusty environment for some species. It shouldn’t be concluded that this is the feather function for all species.

Reflecting or Directing Sound

Wind Dampening

Sensory Feathers, Mechanoreceptors

Filoplume

The filoplume is the third type of feather structure; pennaceous and plumulaceous are the other two. A filoplume is a hair-like feather that has a long bare rachis (shaft) with a small narrow vane at the distal end. The vane consists of one to six short barbs with sparse barbules. They come in various lengths depending on the feather they are supporting.

Ring-necked Pheasant RNEP filoplume

Ring-necked Pheasant RNEP filoplume

Filoplumes grow from their own innervated follicle.1 They are attached to sensory receptors in the skin that detect air pressure, wind and feather movement. Disturbance of a filoplume enlarged tip creates a vibration that is magnified and transmitted by the long, thin shaft to sensory corpuscles at its base.4

Every bird has at least one filoplume per wing, tail or body feather. Flight feathers have up to 12 filoplumes each.3 When grown at the base of a flight feather they are shielded from direct airflow as they do not protrude from under the coverts. They sense the movement, condition, or integration of the companion flight feather.1   

filoplume on flight feather

filoplume on flight feather

An experiment was conducted on a Golden Eagle, where a rectrix was clipped above the calamus. This species molts feathers every two to three years. The clipped feather was replaced prior to the next molt cycle indicating that the filoplume sensed the missing feather.1

Some filoplumes in Pelecaniformes, Procellariformes, and Passeriformes visibly project beyond the surrounding contour feathers.4 A review of bird skins was undertaken to find protruding filoplumes extending beyond the contour feathers. 117 North American oscine species (song birds) were found to have this feature. Most occurrences were found on the nape and less frequently on the crown. It is speculated that they detect airflow, feather ruffling, and drag during flight controlling contour feathers that are out of sight and not accessible to preening.2

ABTO PISI nape filoplumes

Abert’s Towhee ABTO Pine Siskin PISI nape filoplumes

Bristle feathers

Bristle feathers are found on many species. They are whisker-like feathers typically found around the mouth, eyelids and nares. Bristle feathers maybe unbranched or branched. The branched bristles can have minimum barbs at the base to more elaborate structures as found on nightjars. Some birds have both types of structures as shown in the next picture. It was positively correlated that as bristle numbers increase so too the length.8

Abert's Towhee bristles

Abert’s Towhee bristles

Bristles are sensitive to touch and vibrations. They assist in foraging, burrowing nests, obstacle avoidance, protection from airborne particles, and sense airflow. Some websites state that bristles are used to funnel food. That was dispelled in an article about Willow Flycatcher bristles.10 Although feathers are dead cells; the bristle feather sits in an innervated follicle connected by muscle tissue within the dermis. The follicle contains Herbst corpuscles, vibration-sensitive mechanoreceptors, that are sensory nerve endings found only in birds.7

A recent study of bristles states that lower and upper rictal bristles were present in the most recent common ancestor (MRCA ~ 108 mya) of avian phylogeny. This feather is diverse with differences in shape, size and position within the same order, family and genera and is sexually monomorphic. In a sample of 1,000 bird species, about a third had bristles. Nocturnality is a predictor of both the presence and length of the bristle feather.6

It was postulated by Küster in 1905 that bristle feathers may be able to sense sound and used in tracking prey.9

click here to see a collection of bristle feathers

Adornment feathers

Whiskered Auklets have three prominent white face plumes and a longer thin crest on its forehead. The crevice dwelling seabird that is active at night was put through a series of experiments involving a totally dark maze. The experiments indicated the elongated facial feather adornments have a mechanosensory use for orientation. When the plumes were taped down, there was a 275% increase in the head bumping the sides of the maze.4

This Tufted Puffin excavates burrows up to 5 feet deep. Can the assumption be made that it too has mechanosensory plumes?

Tufted Puffin TUPU

Tufted Puffin TUPU

Peafowl Crest

This section is a summary of a study on peafowl crests thus all paragraphs are cited to the same source.11

Studies in mammals and arthropods sensory hairs and antennae have found that they have mechanosensory roles in sound detection. Besides the enervation at the base, mechanical structures and vibrational response play a critical role. These structures are frequency-matched to the stimulus source like a caterpillar sensing an approaching wasp.

Bird feathers, like bristles and filoplumes, are known mechanosensors that enable birds to respond to mechanical stimuli. When sound related, feathers are frequency-tuned to their stimuli providing an advantage to filter irrelevant sounds. These feathers have a resonant frequency, vibrating at the maximum to the energy transferred by the sound wave they are tuned to.   

Tests on peafowl crests demonstrate that they have mechanical properties. The resonant frequency of these feathers matches the frequencies emanating from tail rattling during social displays. The resulting hypothesis is that birds receive and respond to vibrotactile cues in a wider variety of scenarios.

Morphologically Similar

Study Species

 Ind  Common Name  Code  Species  family  order
1 Ostrich OSTR Struthio camelus Struthionidae Struthioniformes
29 Northern Shoveler NSHO Spatula clypeata Anatidae Anseriformes
43 Green-winged Teal GWTE Anas crecca Anatidae Anseriformes
50 Lesser Scaup LESC Aythya affinis Anatidae Anseriformes
64 Common Goldeneye COGO Bucephala clangula Anatidae Anseriformes
77 Gambel’s Quail GAQU Callipepla gambelii Odontophoridae Galliformes
93 Ring-necked Pheasant RNEP Phasianus colchicus Phasianidae Galliformes
98 Chicken CHIC Gallus gallus Phasianidae Galliformes
100 Chukar CHUK Alectoris chukar Phasianidae Galliformes
129 White-winged Dove WWDO Zenaida asiatica Columbidae Columbiformes
131 Mourning Dove MODO Zenaida macroura Columbidae Columbiformes
134 Greater Roadrunner GRRO Geococcyx californianus Cuculidae Cuculiformes
169 Black-chinned Hummingbird BCHU Archilochus alexandri Trochilidae Apodiformes
174 Rufous Hummingbird RUHU Selasphorus rufus Trochilidae Apodiformes
207 Sandhill Crane SACR Antigone canadensis Gruidae Gruiformes
283 Wilson’s Snipe WISN Gallinago delicata Scolopacidae Charadriiformes
520 Cooper’s Hawk COHA Accipiter cooperii Accipitridae Accipitriformes
532 Harris’s Hawk HASH Parabuteo unicinctus Accipitridae Accipitriformes
541 Red-tailed Hawk RTHA Buteo jamaicensis Accipitridae Accipitriformes
545 Barn Owl BANO Tyto alba Tytonidae Strigiformes
547 Flammulated Owl FLOW Psiloscops flammeolus Strigidae Strigiformes
549 Western Screech Owl WESO Megascops kennicottii Strigidae Strigiformes
551 Great Horned Owl GHOW Bubo virginianus Strigidae Strigiformes
554 Northern Pygmy Owl NOPO Glaucidium gnoma Strigidae Strigiformes
555 Ferruginous Pygmy Owl FEPO Glaucidium brasilianum Strigidae Strigiformes
557 Burrowing Owl BUOW Athene cunicularia Strigidae Strigiformes
578 Acorn Woodpecker ACWO Melanerpes formicivorus Picidae Piciformes
579 Gila Woodpecker GIWO Melanerpes uropygialis Picidae Piciformes
603 American Kestrel AMKE Falco sparverius Falconidae Falconiformes
624 Rosy-faced Lovebird RFLO Agapornis roseicollis Psittaculidae Psittaciformes
715 American Crow AMCR Corvus brachyrhynchos Corvidae Passeriformes
811 Curve-billed Thrasher CBTH Toxostoma curvirostre Mimidae Passeriformes
821 European Starling EUST Sturnus vulgaris Sturnidae Passeriformes
840 Hermit Thrush HETH Catharus guttatus Turdidae Passeriformes
841 Cactus Wren CACW Campylorhynchus brunneicapillus Troglodytidae Passeriformes
885 House Sparrow HOSP Passer domesticus Passeridae Passeriformes
948 House Finch HOFI Haemorhous mexicanus Fringillidae Passeriformes
959 Pine Siskin PISI Spinus pinus Fringillidae Passeriformes
960 Lesser Goldfinch LEGO Spinus psaltria Fringillidae Passeriformes
997 Dark-eyed Junco DEJU Junco hyemalis Passerellidae Passeriformes
1017 Abert’s Towhee ABTO Melozone aberti Passerellidae Passeriformes

General Methods

Prior to starting the study I obtained a Scientific Activity License from the state of Arizona that is good for one year and can be renewed. The fee is $70/year. In order to get access to birds from a rehab facility I was requested to get a federal license. The federal license is good for three years at a cost of $100. Both licenses require annual reporting. The possession of feathers and other parts of native North American birds without a permit is prohibited by the Migratory Bird Treaty Act (MBTA). There is no exemption for molted feathers, birds that hit windows or road kill.

Scientific Activity License

Scientific Activity License

Record provenance of each carcass collected which will be used in the annual report.

A note about measurements. All of the measurements are approximate and used to make broad observations. Other than the use of a caliper, ruler measurements round to the nearest millimeter.

Biometrics 

Length of the bird

Morning Dove MODO length

Morning Dove MODO length

Length of the primary

Cactus Wren CACW P10

Cactus Wren CACW P10

Skull volume — length of skull is measured from the base of the maxilla (upper bill) to the back of the  skull. Width and height measured at the maximum of each.

Barn Owl BANO skull

Barn Owl BANO skull

skull volume calculation

skull volume calculation

Meatus area

Ring-necked Pheasant RNEP meatus

Ring-necked Pheasant RNEP meatus

meatus area calculation

meatus area calculation

 

Eye orbit to meatus — the distance from the edge of the eye orbit to the meatus is measured. When working with a northern shoveler I notice that the meatus was significantly further away from the eye than the 3 other, non aquatic, species showed.

An ancillary task is accounting for any bristles. Bristles are an important subset of data for this study.  Their relevance will be explained in the analysis section. 

click here for photos of bristle feathers

Red-tailed Hawk RTHA bristle feathers

Red-tailed Hawk RTHA bristle feathers

Red-tailed Hawk RTHA bristle feathers

Red-tailed Hawk RTHA bristle feathers

Close up picture of the auricular area is taken.

RNEP GIWO auriculars

RNEP GIWO auriculars

If an owl, a picture is taken of the facial disk if a close-up field photo isn’t available. The photo is used to map feather sampling.

Northern Pygmy-Owl NOPO facial feather location

Northern Pygmy-Owl NOPO facial feather location

Pluck auriculars

Auricular feathers were plucked from the closest to meatus out. The plucking stopped when the tip of the next feather no longer covers any part of the meatus.

removing auricular feathers

removing auricular feathers

Feathers are placed on a photo album page in the approximate order they were plucked. Plucked feathers are counted. Some birds have relatively uniform feather length while others have a mixture of short and long feathers. The feather count is subjective, counting large and small feathers (with period in front of the number). 

Acorn Woodpecker ACWO plucked auriculars

Acorn Woodpecker ACWO plucked auriculars

Feather measurements

Sonus feather length and width

chicken sonus auricular

chicken sonus auricular

Barb spacing = rachis length / (number of barbs on one side – 1)

The rachis is measured. The barb pairs are counted in two groups, the shorter ones (S-count) starting at the calamus up to the first full length barb. The longer ones (L-count) are the remainder up to the tip of the feather.

CHIC barb spacing

CHIC barb spacing

Barb angle is measured using the general direction of the barb. The angle will typically be smaller where the barb meets the rachis. Pictures are placed on top of an on-line tool.

https://www.ginifab.com/feeds/angle_measurement/

Chicken CHIC barb angle

Chicken CHIC barb angle

Morphology of the vane, barbs and barbules are noted. Three sets of microscopic views are made of a feather at three magnifications. The first view is of the aftershaft just above the calamus. The magnifications are 50X, 150X, and 300X.

beginning of vane

proximal of vane

The second view is the middle of the vane to the outside edge. This look provides observations on barbule growth. Magnification is at 50X. Note that barbule growth increases towards the distal end of the barbs.

middle of vane

middle of vane

This third view is of the crown of the feather. Note the significant barbule growth. The magnifications are 50X, 150X, and 300X.

end of vane

end of vane

Equipment Used

Macro Photography

Camera — Canon 5D Mark IV

Lens — Canon Macro Lens EF 100mm F2.8

Flash — Macro Ring Light

Remote Switch — SMDV wired remote shutter release

Tripod head — Benrousa 3-way geared head

Tripod — Manfrotto 190X

camera gear

camera gear

Lightpad

AGPtEK light pad

AGPtEK light pad

AGPtEK light pad

Microscopes

Edmund Scientific 300X microscope and reflected light illuminator.

Edmond microscope

Edmond microscope

Amazon WiFi microscope

wifi microscope

WiFi microscope

Micro Photography

iPhone 15 on Edmund microscope

iPhone 15 on Edmund microscope

Dissecting Kit

dissecting kit

dissecting kit

Magnifying work light

Indispensable for close up work. This link is for the latest model.

illuminated-magnifying-lamp

magnifying worklight

magnifying work light

Digital caliper

Home Depot Husky caliper

caliper

caliper

 

Ethics Statement

No birds were killed for this study. Some birds, based on condition, were set out for neighborhood owls. Provenance of the birds is maintained and reported annually (while license is valid) to Arizona Game and Fish Department and U.S. Fish and Wildlife Service. All feathers will be donated to an educational institution upon completion of the study.

Acknowledgements

Ted Bodner — BS Biology, MS Pathology, professional bird guide, professional photographer, mentor for 14 years

TED’S PHOTOGRAPHY WEBSITE 

Laurie Nessel — editor for the Maricopa Audubon Society’s quarterly magazine Cactus Wren-dition and monthly newsletter — published articles, requested support for birds from members.

https://www.maricopaaudubon.org/

Wild at Heart Raptor Rehab facility — supplied 11 raptors and Greater Roadrunner

https://www.wildatheartraptors.org

Damon “Rooster” Cogburn — generously supplied an ostrich, if you live in Arizona take the kids to this wonderful ranch

Rooster Cogburn Ostrich Ranch

References

1 Rohwer, VG, Rohwer, S, Kane, L. (2021) Filoplume morphology covaries with their companion primary suggesting that they are feather-specific sensors. Ornithology 138, 2021, pp. 1–11

2 Clark, Jr., GA and de Cruz, JB. (1989) Functional  interpretation of protruding filoplumes in oscines. The Condor 91:962-965

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

4 Seneviratne, S.S. and Jones, I.L. (2008) Mechanosensory function for facial ornamentation in the whiskered auklet, a crevice-dwelling seabird. Behavioral Ecology · July 2008, 784-790

5 Conover, R. & Miller, E. (1980) Rictal bristle function in willow flycatcher. Condor, 82, 469–471

6 Delaunay, MG, Brassey, C, Larsen, C, Lloyd, H and Grant, RA. (2022) The evolutionary origin of avian facial bristles and the likely role of rictal bristles in feeding ecology. Scientific Reports  (2022) 12:21108

7 Delaunay, M.G., Larsen, C., Lloyd, H., Sullivan, M., and Grant, R.A. (2020). Anatomy of avian rictal bristles in aprimulgiformes reveals reduced tactile function in open-habitat, partially diurnal foraging species. Journal of Anatomy, 237:355–366

8 Delaunay, M.G., Charter, M. & Grant, R.A. (2022) Anatomy of bristles on the nares and rictus of western barn owls (Tyto Alba). Journal of Anatomy, 241, 527–534

9 Lederer, R.J. (1972) The role of avian rictal bristles. The Wilson Bulletin, 84(2), 193–197

10 Conover, R. & Miller, E. (1980) Rictal bristle function in willow flycatcher. Condor 82, 469–471

11 Kane SA, Van Beveren D, Dakin R (2018) Biomechanics of the peafowl’s crest reveals frequencies tuned to social displays. PLoS ONE 13(11): e0207247. 

12 Kosh UR and Wagner H. (2002) Morphometry of Auricular Feathers of Barn Owls. European Journal of Morphology, Vol. 40, No. 1, pp. 15–21

13 Novak, S. (2024) Beasts with Brains. Discover, January 2024, 44-45.

14 West, G. Scaling Biology with Geoffrey West. May 2022, Thinking Tools Podcast #25.

Ancillary Topics

Repurposing

Some of the cadavers received for the study were very fresh. Awareness that many species will eat carrion I placed a dove in the back yard and it was gone the next day. I followed up with a game camera and got some interesting visitors. The first bird I processed was a chicken. I staked it out and found it was visited by neighbors that have lived here for years. The raccoons did not eat from the carcass.

Raccoons

Raccoons

The birds visiting the carrion were also interesting. I’ve been trying to attract a nesting pair of kestrels for over 10 years. I guess I should have been putting out carrion all along.

Repurposing visitors

Repurposing visitors

Browning Trail Camera

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