ornithology Ornithology is a branch of zoology that concerns the "methodological study and consequent knowledge of birds with all that relates to them." Several aspects of ornithology differ from related disciplines, due partly to the high visibility and th ...

, the facial disc is the concave collection of feathers on the face of some

bird Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweig ...

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 In geometry, a paraboloid is a quadric surface that has exactly one axis of symmetry and no center of symmetry. The term "paraboloid" is derived from parabola, which refers to a conic section that has a similar property of symmetry. Every pl ...

that collects sound waves 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 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 very large owl, documented as the world's largest species of owl by length. It is distributed across the Northern Hemisphere, and it is the only species in th ...

has the largest disc of any bird.

References

* Jemima Parry-Jones (2001). ''Understanding Owls: Biology, Management, Breeding, Training''. David & Charles, p. 20. * 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 Bird anatomy Owls