Auriculars and the Sonus Feather
A detailed study of bird auriculars
Feather Series
- A Review of Nomenclature for Avian Color Aberrations
- Feather Basics
- There Are Nine Types of Feathers
- Morphology and Morphometric Characterization of Barn Owl External Sound Related Adaptations
- Auriculars and the Sonus Feather
site under construction
- note that interpretations have not been posted to species level
- page being updated as research progresses
- will note here what new section has been added
- first sections published 3/11/24; dust protector 3/16/24
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. The general consensus states that ear coverts channel sound to the ear and protect the ear from dust.
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.
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.
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.
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
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
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
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?
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.
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
Length of the primary
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.
Meatus area
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
Close up picture of the auricular area is taken.
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.
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.
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).
Feather measurements
Sonus feather length and width
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.
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/
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.
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.
This third view is of the crown of the feather. Note the significant barbule growth. The magnifications are 50X, 150X, and 300X.
Equipment Used
Macro Photography
Lens — Canon Macro Lens EF 100mm F2.8
Remote Switch — SMDV wired remote shutter release
Tripod head — Benrousa 3-way geared head
Lightpad
Microscopes
Edmund Scientific 300X microscope and reflected light illuminator.
Micro Photography
Dissecting Kit
Magnifying work light
Indispensable for close up work. This link is for the latest model.
Digital 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
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
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
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