Facts about Osprey Eyes

Dyfi Osprey Monty with Amber eye

 

 

Fact 21

Osprey are born with black eyes, then they change to blue, and then amber-brown. By the time British ospreys return from Africa as two year olds, they have changed again! Adult ospreys have yellow eyes, apart from Monty because he's special - he still has amber coloured eyes.

We have no idea why Monty does not have the yellow eyes that are normal for adult ospreys. We've asked everybody from geneticists to biologists to professors of vision. If you know, please let us know!

Fact 22

Ospreys, along with most birds, have the largest eyes relative to their size of all animals.

Fact 23

Ospreys have two outer eyelids but these are not used for blinking like ours are. Ospreys sleep by closing the outer eyelids, usually the lower lid that rises up to cover the whole eye.

Fact 24

Ospreys also have a third eyelid called a 'nictitating membrane'. This is a transparent membrane that sweeps horizontally across the eye like a windscreen wiper and helps the eyes remain clean and moist. Ospreys often close their nictitating membrane when they are feeding their chicks to protect their eyes from possible damage from overenthusiastic bobbleheads!

Brenig shows off his nictataing membrane in 2015
and Glesni during a thunder storm in 2016

Fact 25

Ospreys have four types of colour receptors in the eye, we only have three. These help give ospreys the ability to perceive ultraviolet light as well as the wavelengths of light that we see, 'visible light'.

Fact 26

Ospreys have a very high density of receptors in their eyes that give them much better 'visual acuity' than us. Visual acuity is clearness or sharpness of vision - a bit like being able to see the lowest letters of a (Snellen) chart at the opticians from much further away than we can.

Fact 27

Ospreys have binocular vision like we do, having both eyes point forward. However, they have better peripheral vision than we do - being able to see to the sides more.

Monty - peripheral vision
 
Fact 28

Ospreys have dark feathers in front of the eye which help to reduce the glare from the water surface when they are hunting for fish.

Fact 29

Ospreys have the ability to calculate the 'refractive index' of the water they are hunting over and estimate the exact location of the fish including how deep it is. Due to this refractive index, the deeper the fish is, the more it 'appears' to be in a slightly different position. No problem for ospreys!

Fact 30

It is thought that ospreys can 'see' magnetic fields and use these as a navigational guide when migrating. An osprey's right eye contains specialised proteins that perceives the earth's magnetic field, providing the osprey with greater directional information.

Magnetic fields