Blog #281 What’s it like to be an Adult Osprey Part 2: Osprey Vision

What is the most prominent feature you see when you look at an Osprey? I see it’s huge front-facing eyes. Adult Ospreys have brilliant yellow eyes, but they are not born  with this color eye as it is hormonally regulated. The osprey’s eye color changes from  dark blue when first hatched to blood red in nestlings to orange-yellow in juveniles,  which remain like this for their migration to South America. When ospreys return north  to breed at the age of two or three, however, their eyes are bright yellow! 

For birds of prey, including Ospreys, keen eyesight is absolutely critical for finding food,  spotting predators, and generally staying alive. However, Ospreys lead an active life  that could be very hard on the eyes: flying and diving under water at breakneck speeds to capture spiny, struggling fish can inflict damage. 

The translucent nictitating membrane with a bluish tinge covering  

the eyeball of this panting adult Osprey, courtesy of Hellgate Ospreys. 

When an Osprey dives into the water, a protective eyelid—called a nictitating  membrane, third eye, or haw—sweeps over the globe of the eye, closing to protect the 

bird’s eyes like swim goggles. This semi-transparent membrane, also called the plica  semilunaris, membrana nictitans, or palpebra tertia, is a pale, icy-blue, and allows some  areas of light and dark to be seen even underwater. 

Nictitating membranes occur in many birds, reptiles, and even some mammals.  Nictitating membranes protect the eyes from ultraviolet radiation in birds and prevent snow blindness in polar bears. In many species, any stimulus to the eyeball  (such as a puff of air) will result in reflex response of the nictitating membrane, but  Ospreys can actively control their nictitating membrane moving them horizontally across  the eyeball. The membrane is used to moisten and clean the cornea, especially in flight,  and is drawn across the eye when there is a chance the eye might be scratched or  damaged such as when underwater, capturing prey, or feeding their offspring. When  Osprey are diving after fish at 60 mph, they will blink repeatedly to clear debris and  spread moisture across the eyes. 

Preening also requires eye protection because of vigorous pecking and smoothing of  their stiff feathers. Oftentimes, Ospreys also close their white feathered eyelids while preening for extra protection. Scratching is also a time for closed eyes, as the sharp  talons are used to remove debris in the eye area. Ophelia also naps with her eyelids  shut during quiet times at the nest. 

Female Osprey with her white feathered eyelids closed.

Vision is an Osprey’s sharpest sense. While all birds have great eyesight, Ospreys have  even better “eagle eyes” than some eagles, being able to see through the water’s glare.  Avian vision studies have revealed that birds are tetrachromatic having four color detecting and differentiating cells or cones in their retina compared to three in humans,  which enables birds to see in the ultraviolet (UV) spectrum. An Osprey’s excellent visual  acuity and can determine the slightest differences between shades of color, which  humans are unable to detect. This enables the Osprey to detect the slightest contrasts of color in their environment, helping them spot prey. The parts of the retina used in peripheral vision contain a greater number of rod cells, which are more sensitive to light than cone cells, but lack the ability to differentiate color. 

Legendary visual acuity enables the Osprey to hunt for fish  

from 150 ft. in the air, courtesy of Andy Morffew. 

The distance vision of raptors and Ospreys in particular are estimated to be 6–8 times  better than humans, although 4–5 times may be more realistic. The keenness of their  eyesight is due to numerous factors. Raptors have large tubular eyes, giving them a  longer focal length, producing a larger retinal image. In effect, Ospreys have magnifying  eyes with a reduced field of view. The degree of visual acuity depends on the large  number of receptors per square millimeter in the bird’s retina. The more receptors an  animal has, the higher its ability to distinguish individual objects at a distance, especially  when, as in raptors, each receptor is attached to a single ganglion. 

The small depression on the Osprey retina called the fovea is more densely packed with color-detecting cone cells than other parts of the retina. This is where most of their sharply focused highly detailed vision occurs. Ospreys have a second fovea, which makes for an even larger field of sharp focus. 

Anatomy of the avian eye, courtesy of Researchgate.net. Notice the position of the central fovea.

The forward-facing osprey stare provides binocular vision.

The distinctive forward-facing Osprey stare provides binocular vision to the Osprey. The  arrangement of the feathers above the osprey’s eyes serves to reduce glare from the  water surface when the bird is hunting fish. The raptor’s adaptations for optimum visual  resolution has a disadvantage in that its vision is poor in low light level. It is more  difficult for an osprey to fish in low light conditions when overcast or at dusk. 

Ospreys, other birds, reptiles, and their dinosaur ancestors have small boney plates that encircle the eye cavity called a sclerotic ring. This ring, which adds a menacing look to skulls, helps provide protection and support for the large raptor eyes and creates additional anchors for muscles that help with focusing and blinking. For close up vision, their unique Crampton’s muscles (see Anatomy of the Avian Eye, above) apply pressure, reshaping the cornea, and allowing them to have a much sharper focus than we do. 

Red Tail Hawk skull showing the sclerotic ring, courtesy of DeLoy Roberts. 

How do Osprey hunt for underwater prey? When light waves travel from air into water, it  slows down, causing it to change direction slightly, which is called refraction. It causes a  fish in the water to appear closer than they really are. Despite this, Ospreys are  excellent anglers and average catching a fish at least one in every four dives,  with success rates sometimes as high as 70%.

Light is bent or refracted as it passes through two different media with different densities, such as air and water. courtesy of freepic.com. 

When viewed from the air, a fish in the water will appear closer  

than it actually is, courtesy of calacademy. 

Eyes to the sky!  

Candace  

Candace E. Cornell  

Friends of Salt Point  

Lansing, NY  

cec222@gmail.com

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