Black Eagle Project Roodekrans

12 th September 2013 - # 2

Our second weekly eagle info page – if there is anything you would like to have clarified and or discussed, give me a shout on 079 505 6419 or send me an email on [email protected]

We...well the eagles...are in their 5 th week of incubation – 28 days on Thursday 12 th September – or 30 and 31 days on Saturday and Sunday respectively.

All looks well although the male appears a tad lazy to go hunting, preferring to stay at home harassing the pied crows and finding every opportunity available to him to get out of incubation to chase the blighters away! During my shift last Sunday morning, they were mating on “The Branch”, which is not unusual activity and is referred to as pair bonding. Having already laid two eggs on the lower nest, there is simply no way that they will lay another clutch on the 2 nd nest! ☺ But then again...this is Roodekrans and anything can happen...

Estimated time of hatching is on Saturday 28 th September – 44 days – incubation is now over the halfway mark and the eagles are on the downhill slide...heading for the finish line…

The Eagle’s

The eagle eye is among the strongest in the animal kingdom, with eyesight estimated at 4 to 8 times stronger than that of the average human. An eagle is said to be able to spot a rabbit 3.2 km away. Although an eagle may only weigh 4.5 kg, its are roughly the same size as those of a human. As the eagle descends from the sky to attack its prey , the muscles in the eyes continuously adjust the curvature of the eyeballs to maintain sharp focus and accurate perception throughout the approach and attack . In addition to eagles, birds such as hawks, falcons, and robins have extraordinary vision which enable them to gather their prey easily. Their eyes are stated to be larger in size than their brain, by weight. Colour vision with resolution and clarity are the most prominent features of eagles' eyes, hence sharp-sighted people are sometimes referred to as "eagle-eyed.” Eagles can identify five distinctly-coloured squirrels and locate their prey even if hidden.

Eagle weight varies: a small eagle could weigh 0.7 kilograms, while a larger one weighs 6.5 kilograms; an eagle of about 10 kilograms weight could have eyes as big as that of a human being who weighs 90 kg). Although the size of the eagle eye is about the same as of a human being, the back side shape of the eagle eye is flatter. An eagle's allows for a higher Nyquist limit. Its retina is more pronounced with rod cells and cone cells. In the eagle, the retina's fovea has one million cells per mm as compared to 200,000 per mm in humans. Eagles have a second fovea and three eyelids (two of which are visible). The second fovea in eagles gives them better and sharper vision while the long, narrow ribbon-shaped area that connects the two eagle foveas is inferred to be a third fovea. The phenomenon of an eagle turning its flexible head almost 270 degrees, while sitting or flying, is attributed to the fact that when its large head is turned fully its eyes are also turned, unlike a human.

An eagle in flight can reputedly sight a rabbit two miles away. Talon – eye coordination is a hunting imperative. From its perch at the top of trees, the eagle can dive at speeds of 201–320 km/h to catch its prey by its talons. Eagles, in their young age, cannot locate fish below water as a result of refraction error of the eye, so they compensate by grabbing dead fish floating on the surface. As they grow older, the refraction error naturally rectifies itself and they are able to spot fish below the surface. The fierce look of the eagle is due to the placement of a bony ridge above its eyes, the small amount of bare skin between its eyes, and its sharp beak. The feathers on its body generally do not grow over the eyes. The ridge protects the eyes from protruding tree branches when it perches on trees, and also from prey which struggles to escape. Each eagle eyeball moves separately. The eyeball is so large and so tightly fit that the eagle can barely turn it within the socket. That the eyes are located in front of its head with face forward and looking slightly askew is an advantage. Though its hearing does not match its eye power, mating calls are said to be heard for several miles.

Eagles blink up as their top eyelid is larger than the bottom one; the additional inner eyelid is known as a nictitating membrane, which “grows in the inner cornea of the eye, next to the tear duct”. Eagle tears moisten the eyes and contain the chemical lysozyme which protects against salt water and also destroys bacteria, thus preventing eye infections. The nictitating membrane acts a sweeping wiper across the eye. The eagle is a pale yellow colour, much lighter than human eyes. Both eagles and humans have a white area called the , but in the case of eagles, it is hidden below the eyelid. Eyelid openings are oval-shaped in humans, while they are round in the case of birds' eyes. The precise adjustment and focus in an eagle's eyes is attributed to a changing shape of the lens and corneas, a feature not found in human beings. Most eagles have excellent vision. Generally, eagles do not suffer from (near-sightedness) and hyperopia (farsightedness); those who have these defects cannot hunt easily and eventually starve to death. Eagles have the unique feature of the pectin. Its function is not clearly understood, but the general belief is that it helps to nourish the retina, keeps it healthy without blood vessels, facilitates the fluids to flow through the vitreous body at an appropriate pressure, absorbs light to minimize any reflections inside the eye that could impair vision, helps perceive motion, creates a protective shade from the sun, and magnetic fields.

Compiled by:

Bo van der Lecq