Introduction
Being largely nocturnal in their habits, owls face a different set of challenges to many other predatory birds. In addition to their excellent vision, hunting owls also rely on their sense of hearing to find and locate prey. Owl hearing differs from that of humans in several ways, not least in terms of the structure of their ear. Understanding how owl hearing works tells us more about how they can find and catch prey in near darkness.
About owl hearing
The ears of an owl lack any obvious external structure and are hidden within the feathers that make up the ruff that surrounds the facial disc. The feathers of the ruff are thickened and, coupled with the filamentous ‘auricular’ feathers of the facial disc, help to channel sound towards the ears. Interestingly, the two ears are asymmetrical in their positioning in most owl species, the left ear positioned lower than the right, and the two also out of line in the vertical plane. Such asymmetry generates a tiny amount of separation between when a sound hits one ear compared to the other; this allows an owl to better pinpoint the source of a sound than is the case with human hearing and our symmetrically placed ears.
In a few owl species the asymmetry in ear position can also be seen in the structure of the skull, as this illustration of a Tengmalm’s Owl skull demonstrates.
Ear structure
In common with the mammalian ear, the ear of an owl has three recognisable regions: the outer ear, middle ear and inner ear. Sound reaches the ear in the form of pressure waves, which are then channelled towards the ear drum, which is located within the middle ear. The ear drum is proportionately larger in owls than it is in other birds. Through the ear drum and a single bone, the airborne sound waves are then transformed into fluid-borne vibrations which trigger thousands of minute hairs within the inner ear. Sounds of different frequencies reach different parts of the inner ear and trigger different hairs to move, the resulting signals building up a complex picture of the sounds being received.
How good is owl hearing?
The sensitivity of owl hearing is not that dissimilar from our own, and there is a certain amount of overlap between the hearing abilities of individual people and individual owls. Owl hearing is certainly more sensitive than that of other birds, particularly at frequencies of 5 kHz and above. Barn Owls have been shown to use sound frequencies above 8.5 kHz to direct and make an accurate strike at a prey item. A hunting owl, therefore, will use the calls and movements made by a mouse, vole or shrew to direct its strike.
Some authors have suggested that owl hearing might be so sensitive as to enable an owl to hunt and catch prey in complete darkness. The experimental work that has been carried out to test this does demonstrate an ability for certain species of owl to do this. An owl in a darkened aviary first turns its head towards the source of a sound before making a strike. However, it appears that the owls in these experiments were successful in their strikes only if they had first had the opportunity to use visual cues alongside auditory ones – i.e. the owls had been allowed to hunt in the experimental aviary with the lights on, before later hunts were made in darkness.
The importance of hearing to a Barn Owl, quartering over rough grassland, can be seen in the way that it will pause in its flight before making a strike. This suggests it first hears a mouse or vole in the vegetation and then checks its flight to better position its strike. Hearing may be even more important for a species like Great Grey Owl, which is sometimes seen to take small mammals active under a layer of snow. In such a case there is no chance that the owl could have seen the small mammal, so it must have been relying on its hearing instead.
