Facial disc

Harriers, such as this Australasian Harrier, *Circus approximans*, have a "facial ruff" around the neck that can be raised in response to sounds.

In ornithology, the facial disc is the concave collection of feathers on the face of some birds—most notably owls—surrounding the eyes. The concavity of the facial disc forms a parabola that collects sound waves and directs those waves towards the owl's ears. This collection of feathers heightens the bird's sensory perception, 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 has the largest disc of any bird.

References

Jemima Parry-Jones (2001). Understanding Owls: Biology, Management, Breeding, Training. David & Charles, p. 20. ISBN 0715312235
Robert E. Simmons (2000). Harriers of the World: Their Behaviour and Ecology. Oxford University Press, pp. 53–56. ISBN 0198549644