ornithology Ornithology, from Ancient Greek ὄρνις (''órnis''), meaning "bird", and -logy from λόγος (''lógos''), meaning "study", is a branch of zoology dedicated to the study of birds. Several aspects of ornithology differ from related discip ...

, the facial disc is the concave collection of

feather Feathers are epidermal growths that form a distinctive outer covering, or plumage, on both avian (bird) and some non-avian dinosaurs and other archosaurs. They are the most complex integumentary structures found in vertebrates and an exa ...

s on the face of some

bird Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...

s—most notably

owl Owls are birds from the order Strigiformes (), which includes over 200 species of mostly solitary and nocturnal birds of prey typified by an upright stance, a large, broad head, binocular vision, binaural hearing, sharp talons, and feathers a ...

s—surrounding the eyes. The concavity of the facial disc forms a circular paraboloid that collects

sound wave In physics, sound is a vibration that propagates as an acoustic wave through a transmission medium such as a gas, liquid or solid. In human physiology and psychology, sound is the ''reception'' of such waves and their ''perception'' by the ...

s and directs those waves towards the owl'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 harriers, 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

barn owl The barn owls, owls in the genus '' Tyto'', are the most widely distributed genus of owls in the world. They are medium-sized owls with large heads and characteristic heart-shaped faces. They have long, strong legs with powerful talons. The ter ...

has the most visually prominent facial disc, measuring about 110 mm (Simmons), while the

great grey owl The great grey owl (''Strix nebulosa'') (also great gray owl in American English) is a true owl, and is the world's largest species of owl by length. It is distributed across the Northern Hemisphere, and it is the only species in the genus ''Str ...

has the largest disc of any bird. Due to shared

inner ear The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In mammals, it consists of the bony labyrinth, a hollow cavity in the ...

anatomy with barn owls, it is theorized that the

feathered dinosaur A feathered dinosaur is any species of dinosaur possessing feathers. That includes all species of birds, and in recent decades evidence has accumulated that many non-avian dinosaur species also possessed feathers in some shape or form. The exte ...

'' Mononykus'' may also have had a facial disc.

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 '' Strigiformes'' (all owls). 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. The five separate instances discovered so far include ''

Tyto ''Tyto'' is a genus of owls in the family Tytonidae. Depending on the species and the locality, common names include barn owl, common barn owl, grass owl, sooty owl, masked owl, field owl or simply owl. It is the most widely distributed genus o ...

'' (barn owls, grass owls, and masked owls) and ''

Phodilus The bay owls (''Phodilus'') are a genus of Old World owls in the family Tytonidae. The defining characteristics of bay owls are their smaller bodies, in comparison to the barn owls ''Tyto'' in the same family, and their U- or V-shaped faces. The ...

'' (bay owls) in the family ''

Tytonidae The bird family Tytonidae, which includes the barn owls ''Tyto'' and the bay owls ''Phodilus'', is one of the two Family (biology), families of owls, the other being the true owls or typical owls, True owl, Strigidae. They are medium to large owl ...

'', and ''

Bubo A bubo (Greek βουβών, ''boubṓn'', 'groin') is adenitis or inflammation of the lymph nodes and is an example of reactive infectious lymphadenopathy. Classification Buboes are a symptom of bubonic plague and occur as painful swellings in ...

'' and '' Strix'' (horned owls, eagle owls, fish owls, and earless owls), ''

Asio ''Asio'' is a genus of typical owls, or true owls, in the family Strigidae. This group has representatives over most of the planet, and the short-eared owl is one of the most widespread of all bird species, breeding in Europe, Asia, North Ameri ...

'' (eared owls), and '' Aegolius'' (saw-whet owls) in the family ''

Strigidae The true owls or typical owls (family (biology), family Strigidae) are one of the two generally accepted families of owls, the other being the barn owls and bay owls (Tytonidae). This large family comprises 230 living or recently extinct species ...

''. In addition to those five, the genus ''Strix'' in particular is suspected to include multiple independent instances of facial disc evolution, indicated by significant ear structure differences between species within that genus that exceeds the differences within other owl genera. 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. 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

and

snowy owl The snowy owl (''Bubo scandiacus''), also known as the polar owl, the white owl and the Arctic owl, is a large, white owl of the true owl family. Snowy owls are native to the Arctic regions of both North America and the Palearctic, breeding mo ...

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. 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.

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. 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. 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. 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. In the

, 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.

Skull

The presence of the facial disc in owls, which acts like a component of the ear analogous to a human auricle, goes hand in hand with the unique ear structure beneath it. The symmetrical facial disc hides asymmetrical ear orientations underneath the feathers, where the two ears point at different angles. 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. 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

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. Like owls, this consists of both a facial disc and a facial ruff. 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. 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. This might be an example of

convergent evolution Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last comm ...

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. 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. 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. It is also possible that harrier facial discs and ruffs are

vestigial Vestigiality is the retention, during the process of evolution, of genetically determined structures or attributes that have lost some or all of the ancestral function in a given species. Assessment of the vestigiality must generally rely on co ...

References