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 was special - he still had amber coloured eyes.
We have no idea why Monty did 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!
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.
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.