Birds of prey, otherwise known as raptors, can be found on every continent bar Antarctica. There are over 500 recognised species of raptor, generally divided into Falconiformes and Strigiformes with further subdivisions, and all are classified as carnivorous (Britannica, 2023). Birds of prey have evolved unique solutions in relation to their eyes to assist with their behaviour – they are an integral component of what makes a raptor a raptor.
Why are raptor eyes different?
Birds’ eyes are comparatively large for their size, ranging from 25 percent to almost 70 percent of their skull size (Pollock and Murphy, 2010). For comparison, an eagle’s eyes are about the same size as a human’s, which, when you consider that the average weight of a native golden eagle (5kg) is 17 times less than the average weight of a UK adult male, is a large eye to skull size ratio.
Position and shape
Eyes of all raptor species are forward facing, though owls’ are more prominently located on the front of the face. Due to the size of their eyes, there is a reduction in muscles to allow eye movement – this is why raptors turn their heads to look around. A sclerotic ring of bone holds the eye in place in the skull, maintaining the size and shape of the eyes.
Forward-facing eyes have much greater depth perception; however, this is at the cost of overall field of vision (Figure 1). In coevolutionary terms, a predatory species often evolves binocular vision to better spot and gauge depth when hunting prey. In comparison, a prey species will more likely evolve a more monocular vision to assist in spotting predators and evading predation.
Raptors are either active hunters, carrion feeders or, as in many species, a bit of both. As such, they require keen eyesight to locate prey, whether alive or deceased. Raptors have evolved binocular vision so can focus both eyes on a single focal point.
The generally accepted rule is that on average birds of prey have around six-times better eyesight than a human. Some species are not as perceptive while others are more so, with the forementioned eagles having a generally accepted eight-times better eyesight than people. This means prey items can be spotted at great distances and when the raptor is at high altitude.
The generally accepted rule is that on average birds of prey have around six-times better eyesight than a human
Birds of prey also have excellent colour vision; in fact, we have been aware that birds are able to see ultraviolet light since the early 1990s. The colour in which raptors see is crucial, with owls being able to see clearly in black and white at night; diurnal raptors, in comparison, generally have good colour vision and can see some colours in the UV spectrum (Fite, 1973).
Eye colour is indicative of their time of activity, with nocturnal raptor species largely having darker eyes than those active during the day. However, it is worth noting that age-related changes can occur when young birds reach sexual maturity.
The retinal structure of birds with regard to the arrangement of retinal layers is the same as seen in other vertebrates; however, key areas differ.
For raptors, the number of rods and cones in the retina is greater than those seen in most mammals, but species-dependent factors result in differences in the levels of cones and rods seen between raptor species (Pollock and Murphy, 2010).Though there are diurnal hunting exceptions, owls are largely considered nocturnal by nature, and adaptations can be seen for this behaviour: a larger portion of rods are seen as indicative of their nocturnal habits and requirement for increased light sensitivity. By contrast, cone photoreceptors are higher in diurnal species as they hunt during the day, therefore they generally have access to more light (Ruggeri et al., 2010)(Figure 2).
The areas where these adaptions in rods and cones are found are known as the foveae. In diurnal raptors, two foveae are present, with the central region deeper and containing a superior concentration of photoreceptors, which allow greater bifocal vision (Ruggeri et al., 2010). Comparatively, owls only have one fovea, found in the temporal region. This indicates their reduced ability to focus on prey from a greater distance due to light restrictions. However, they have adapted much greater hearing ability to compensate for this.
The number of rods and cones in the retina is greater than those seen in most mammals, but species-dependent factors result in differences in the levels […] seen between raptor species
Owls have tubular shaped eyes as they have evolved larger pupils and corneas for increased light intake, while diurnal raptor species have more spherical eyes as they have focused on greater binocular vision (Kiama et al., 2001).
Regardless of preference for hunting times, all bird retinas are avascular, allowing the structure to provide sharp vision by minimising the light scattering between the retinal surface and photoreceptors. By removing the blood vessels in front of the retina, birds have evolved a unique solution to sharper imagery visualisation: nourishment is supplied by the pecten oculi and oxygenation comes from the choriocapillaris. The optic nerve head is where the pecten starts and it terminates in the vitreous body.
Ophthalmic exams for trauma-related injuries in raptors
Eyes are essential to a raptor, and a competent ophthalmic examination must be performed in any suspected trauma cases, whether the patient is owned or wild. This is because haemorrhage of the choroid and/or pecten into the vitreous body occurs in approximately one-third of traumatised raptors (Coles, 1997) and can easily be missed if a suitable examination is not conducted.
The easiest form of assessment for trauma-related injury in raptors is via an ophthalmic exam. Initially assess the raptor’s eyesight from a distance, discerning their reaction to stimuli and responses, etc. Once completed, use of an ophthalmoscope will allow the internal structures to be assessed and any damage specific to the pecten to be reviewed (Korbel, 2000).
Haemorrhage of the choroid and/or pecten into the vitreous body occurs in approximately one-third of traumatised raptors and can easily be missed if a suitable examination is not conducted
The chance of a successful outcome following damage to a bird of prey’s eye is varied and dependent on the individual. However, when there is significant pecten damage, often the outcome will not be favourable.