Eagle Eyed

Visual Acuity

White-tailed Eagles have a highly developed sense of sight. Their extremely sharp eyesight enables them to locate prey from a very long distance. The eyes of a White-tailed Eagle are not only large in relation to their head, but larger than humans.

The diameter of a White-tailed Eagle’s eye measures 46 mm (4.6 cm) and is one of the largest eyes of any bird in the world relative to its body size. For comparison, the diameter of the objective lens in a pair of 10 x 42 binoculars that are widely used to watch eagles is only 42 mm and the human eye averages just 24 mm!

It is said that an eagle’s sharpness of vision is up to six times as great as a man with perfect eyesight, although this acuity has a tendency to vary depending on which author you read and believe. What is without question is the fact that the White-tailed Eagle possesses extremely good eyesight!

The eyes of a White-tailed Eagle are situated at the side of its skull and have two centres of focus, allowing them to see both forwards and sideways at the same time. This overlapping of vision gives the White-tailed Eagle binocular eyesight which helps them gauge distances more accurately.

Like other birds, the retina of a White-tailed Eagle is made up of cones (to recognise colour) and rods (to distinguish light). The eye of a White-tailed Eagle has more cones than rods, as is befitting of a diurnal raptor. Day-flying birds of prey have a tendency to have pale-coloured eyes, whereas the irides of nocturnal hunters are dark. This demonstrates the individual species balance of cones and rods in the retina. It is perhaps obvious to state that a White-tailed Eagle does not have to be able to see as well at night as a Tawny Owl may!

Like the pale head and white tail of adulthood, the eyes of White-tailed Eagles lighten with age. Immature birds have dark, brown eyes, as illustrated in the photograph, above right.

Olfactory Perception

Most birds possess rather poor or limited olfactory perception of their surroundings. There is nothing to suggest that White-tailed Eagles are any different, although it is interesting to speculate that they could have an enhanced sense of smell which may help them find food, such as deer or sheep carrion.

Turkey Vultures Cathartes aura are a common and widespread bird of prey in North America. In addition to their excellent eyesight, they have a well developed sense of smell, both of which are used to find food.

Not being a true eagle, but one with strong vulturine affinities, it is understandable that some should question whether the White-tailed Eagle enjoys a similar olfactory prowess when it comes to locating its prey on the Isle of Mull. However, as interesting as it may be, there is no scientific evidence to suggest that it does.