imported>Mucketymuck: As intro notes, such discs aren’t present only in owls. Adjective misused as preposition also corrected.

2025-06-20T22:03:14Z

As intro notes, such discs aren’t present only in owls. Adjective misused as preposition also corrected.

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{{Short description|A concave collection of feathers surrounding the eyes of some birds, most notably owls}}
[[File:Tyto alba close up.jpg|thumb|The [[Western barn owl|barn owl]], ''Tyto alba'', has a prominent heart-shaped facial disc that aids it during hunting.]]

In [[ornithology]], the '''facial disc''' is the concave collection of [[feather]]s on the face of some [[bird]]s—most notably [[owl]]s—surrounding the eyes. The [[wikt:concave|concavity]] of the facial disc forms a [[paraboloid|circular paraboloid]] that collects [[sound wave]]s and directs those waves towards the bird's ears. The feathers making up this disc can be adjusted by the bird to alter the focal length of this sound collector, enabling the bird to focus at different distances and allowing it to locate prey by sound alone under snow, grass, and plant cover.

Other bird species, such as [[Harrier (bird)|harrier]]s, have less prominent facial discs. In harriers, the related term '''facial ruff''' refers to feathers around the neck that are raised in response to noise, essentially enlarging the facial disc and improving hearing.

The [[Western barn owl|barn owl]] has the most visually prominent facial disc, measuring about 110 mm (Simmons), while the [[great grey owl]] has the largest disc of any bird.

Because of its shared [[inner ear]] anatomy with barn owls,<ref>{{Cite journal |last1=Choiniere |first1=Jonah N. |last2=Neenan |first2=James M. |last3=Schmitz |first3=Lars |last4=Ford |first4=David P. |last5=Chapelle |first5=Kimberley E. J. |last6=Balanoff |first6=Amy M. |last7=Sipla |first7=Justin S. |last8=Georgi |first8=Justin A. |last9=Walsh |first9=Stig A. |last10=Norell |first10=Mark A. |last11=Xu |first11=Xing |last12=Clark |first12=James M. |last13=Benson |first13=Roger B. J. |date=2021-05-07 |title=Evolution of vision and hearing modalities in theropod dinosaurs |url=https://ora.ox.ac.uk/objects/uuid:c9faa76c-de6a-470f-bfc4-0aaf82bda030/files/sx633f165b |journal=Science |language=en |volume=372 |issue=6542 |pages=610–613 |doi=10.1126/science.abe7941 |pmid=33958472 |bibcode=2021Sci...372..610C |issn=0036-8075}}</ref> it is theorized that the [[feathered dinosaur]] ''[[Mononykus]]'' may also have had a facial disc.<ref>{{Cite web |date=23 May 2022 |title='Prehistoric Planet,' a Stunning New Series, Depicts Dinosaurs Like Never Before {{!}} Audubon |url=https://www.audubon.org/news/prehistoric-planet-stunning-new-series-depicts-dinosaurs-never |access-date=2024-05-16 |website=www.audubon.org |language=en}}</ref>

== Evolution of facial disc anatomy ==

=== In owls ===

Facial discs and associated underlying ear structures are thought to have evolved at least five separate times within the order of ''[[Owl|Strigiformes]]'' (all owls).<ref name="TestBook" /> Furthermore, since that conclusion was reached without examining every genus within ''Strigiformes'', it remains possible that there may also be additional instances of facial disc evolution in owls that have not yet been catalogued.<ref name="Norberg Ocurrence" /> The five separate instances discovered so far include ''[[Tyto]]'' (barn owls, grass owls, and masked owls) and ''[[Phodilus]]'' (bay owls) in the family ''[[Tytonidae]]'', and ''[[Horned owl|Bubo]]'' and ''[[Strix (bird)|Strix]]'' (horned owls, eagle owls, fish owls, and earless owls), ''[[Asio]]'' (eared owls), and ''[[Aegolius]]'' (saw-whet owls) in the family ''[[Strigidae]]''. In addition to those five, the genus ''Strix'' in particular is suspected to include multiple independent instances of facial disc evolution,<ref name="Norberg Ocurrence" /> indicated by significant ear structure differences between species within that genus that exceeds the differences within other owl genera.<ref name="TestBook" />

The facial disc and associated ear anatomy are thought to have evolved in response to selection pressures accompanying the shift to owls hunting primarily using sound.<ref name="Norberg Ocurrence" /> Specifically, the facial disc probably came about as owls transitioned to hunting mainly by hearing after hunting mainly by sight became more difficult. Factors that may have influenced the evolution of facial discs and asymmetrical ear placement include owls living in habitats with dense vegetation or snow cover (the [[great grey owl]] and [[snowy owl]] being examples of the latter), owls hunting at night, and owls hunting ground-based prey that are obscured by foliage as opposed to more readily visible avian prey.<ref name="TestBook" /> All of these factors make visual-based hunting more difficult, and so might have driven selection for adaptations that help with hunting by sound instead, such as the facial disc to help the owl hear its prey, as well as specialized silent wing feathers so the prey does not hear an approaching owl.<ref>{{cite journal |last1=Clark |first1=Christopher J |last2=LePiane |first2=Krista |last3=Liu |first3=Lori |title=Evolution and Ecology of Silent Flight in Owls and Other Flying Vertebrates |journal=Integrative Organismal Biology |date=1 January 2020 |volume=2 |issue=1 |pages=obaa001 |doi=10.1093/iob/obaa001|pmid=33791545 |pmc=7671161 }}</ref>

==== Feathers ====

The feathers of the facial disc are transparent to sound, to allow sound to pass through the disc to the ears below, whereas the stiffer facial ruff feathers around the rim of the disc reflect sound, directing it towards the face.<ref name="Campenhausen" /> Unfortunately, it is nearly impossible to trace the evolution of these specialized feather types, or of any feather evolution in general, given that the rarity of feathers in the fossil record severely hampers gathering of knowledge about any bird species that are not currently living.<ref>{{cite journal |last1=Prum |first1=Richard O.|last2=Brush |first2=Alan H.|title=The Evolutionary Origin and Diversification of Feathers |journal=The Quarterly Review of Biology |date=September 2002 |volume=77 |issue=3 |pages=261–295 |doi=10.1086/341993|pmid=12365352 }}</ref> But while it may be difficult to ascertain when a given feather type evolved, it is possible to make an educated guess on what other kind of feathers it might have evolved from, via comparison of feather structures at the microscopic level. This method yields the conclusion that in barn owls, feathers in the facial disc may have evolved from contour feathers, are the feathers that cover the owl’s body and the back of its head.<ref name="Koch and Wagner" /> The open structure of the feathers that comprise the facial disc represent a simplification of the contour feather structure, rather than an increase in complexity.<ref name="Koch and Wagner" />

In the [[Western barn owl|barn owl]], an owl species with one of the most conspicuous facial discs, the uniform white color of the feathers in the facial disc are thought to a function only for sound collection rather than display. (See first image above.) Closely spaced barbs on feathers located directly over the ear openings may also function as barriers to protect the ear.<ref name="Koch and Wagner" />

==== Skull ====
[[File:TengmalmSkull.jpg|thumb|Figure 6 in the bottom right corner of this image is a head-on view of a [[Boreal owl|Boreal Owl]]’s skull, showing asymmetrical structures that direct the ear openings in different directions to assist in precise hearing.]]
The presence of the facial disc in owls, which acts like a component of the ear analogous to a human [[Auricle (anatomy)|auricle]], goes hand in hand with the unique ear structure beneath it.<ref name="Campenhausen" /> The symmetrical facial disc hides asymmetrical ear orientations underneath the feathers, where the two ears point at different angles.<ref name="Norberg skull" /> It is not the feathers or the ears themselves but rather the asymmetrical shape of the owl’s skull that causes the ears to be oriented this way. (See Figure 6 in bottom right corner of the image at right.) Asymmetrically oriented ears enable an owl to determine where a sound is coming from without tilting its head by comparing the different sounds received by each ear; the facial disc further enhances this sound collection.<ref name="TestBook" /><ref name="Norberg skull" /> When studying the evolution of owl hearing, the distinctive skull shape of owls, indicative of asymmetrical ears, is used as a proxy for the presence of an accompanying facial disc, since the facial disc and ear structure work together to improve hearing, and presumably evolved in tandem in response to the same selection pressures.

=== In harriers ===

[[Harrier (bird)|Harriers]] (birds of prey in the genus ''Circus'') have a similar arrangement of feathers on their faces, which gives them superior hearing compared to related diurnal raptors.<ref>{{cite journal |last1=Terrill |first1=Ryan S. |last2=Shultz |first2=Allison J. |title=Feather function and the evolution of birds |journal=Biological Reviews |date=2023 |volume=98 |issue=2 |pages=540–566 |doi=10.1111/brv.12918|doi-access=free |pmid=36424880 }}</ref> Like owls, this consists of both a facial disc and a facial ruff.<ref name="Harrier book" /> While harriers’ facial discs are too small to give them the same precise hearing as owls’ facial discs, this discrepancy can be offset by harriers’ facial ruffs, which harriers can extend to enhance their hearing when necessary.<ref name="Harrier book" /> Like harriers, the feathers in the facial ruff in the barn owl have also been found to contribute to the channeling of sound towards the ears in addition to the facial disc.<ref name="Campenhausen" /> This might be an example of [[convergent evolution]] between owls and harriers, since harriers and owls independently evolved facial ruffs, but the facial ruff shares the same structure across both groups and appears to serve the same function as well.

The evolutionary pressures that brought about harrier facial discs and facial ruffs are not fully understood, though some tentative conjectures have been proposed.<ref name="Harrier book" /> In contrast to most owl species, harriers hunt during the day, so low vision at night is unlikely to have driven selection for better hearing.<ref name="Harrier book" /> It is thought that hunting ground-based prey in thick vegetation played a role instead. Larger and more extensive facial discs and ruffs in harrier species that primarily go after terrestrial prey (such as the [[hen harrier]]), compared to smaller, incomplete facial discs and ruffs in harrier species that more commonly hunt birds and insects (such as European populations of [[Montagu's harrier]]), have been cited as evidence for this hypothesis.<ref name="Harrier book" /> It is also possible that harrier facial discs and ruffs are [[Vestigiality|vestigial]].<ref name="Harrier book" />

==See also==
*[[Sound localization in owls]]

== References ==
{{reflist|refs=
<ref name="TestBook">{{cite book |title=Ecology and conservation of owls |date=2002 |publisher=CSIRO Pub |isbn=9780643067943}}</ref>
<ref name="Campenhausen">{{cite journal |last1=von Campenhausen |first1=Mark |last2=Wagner |first2=Hermann |title=Influence of the facial ruff on the sound-receiving characteristics of the barn owl's ears |journal=Journal of Comparative Physiology A |date=1 October 2006 |volume=192 |issue=10 |pages=1073–1082 |doi=10.1007/s00359-006-0139-0|pmid=16721575 }}</ref>
<ref name="Koch and Wagner">{{cite journal |last1=Koch |first1=U. R. |last2=Wagner |first2=H. |title=Morphometry of auricular feathers of barn owls (Tyto alba) |journal=European Journal of Morphology |date=2002 |volume=40 |issue=1|pages=15–21 |doi=10.1076/ejom.40.1.15.13957 |pmid=12959344 }}</ref>
<ref name="Norberg skull">{{cite journal |last1=Norberg |first1=R. Å. |last2=Boycott |first2=Brian Blundell |title=Skull asymmetry, ear structure and function, and auditory localization in Tengmalm's owl, Aegolius funereus (Linné) |journal=Philosophical Transactions of the Royal Society of London. B, Biological Sciences |date=January 1997 |volume=282 |issue=991 |pages=325–410 |doi=10.1098/rstb.1978.0014}}</ref>
<ref name="Harrier book">{{cite book |last1=Simmons |first1=Robert E. |title=Harriers of the world: their behaviour and ecology |date=2000 |publisher=Oxford University Press |location=Oxford |isbn=9780198549642}}</ref>
<ref name="Norberg Ocurrence">{{cite journal |last1=Norberg |first1=R. Å. |last2=Boycott |first2=Brian Blundell |title=Ocurrence [sic] and independent evolution of bilateral ear asymmetry in owls and implications on owl taxonomy |journal=Philosophical Transactions of the Royal Society of London. B, Biological Sciences |date=January 1997 |volume=280 |issue=973 |pages=375–408 |doi=10.1098/rstb.1977.0116}}</ref>}}

* Jemima Parry-Jones (2001). ''Understanding Owls: Biology, Management, Breeding, Training''. David & Charles, p. 20. {{ISBN|0-7153-1223-5}}
* Robert E. Simmons (2000). ''Harriers of the World: Their Behaviour and Ecology''. Oxford University Press, pp. 53–56. {{ISBN|0-19-854964-4}}
* U.R. Koch, H. Wagner (2002). ''Morphometry of Auricular Feathers of Barn Owls (Tyto alba)''. European Journal of Morphology, Vol. 40, No. 1, pp. 15–21

[[Category:Bird anatomy]]
[[Category:Owls]]
[[Category:Auditory system]]

Facial disc - Revision history

imported>Mucketymuck: As intro notes, such discs aren’t present only in owls. Adjective misused as preposition also corrected.