Alright, I’ve done some digging around various journals of science and books and so on and this is what I’ve determined:
Eagles do not have stereoscopic vision due to the placement of their eyes; however they do have some of the best vision in nature and are capable of amazing things (studies suggest that some eagles can spot an animal the size of a rabbit up to two miles away).
Now, if these birds don’t have stereoscopic vision then there must be some sort of system they use to gauge distance unrelated to the structure of their eyes.
I have found out the following about eagle eye structure in general which is: " In part, eagles have excellent vision because their eyes, which are very large in proportion to their heads, are densely packed with sensory cells. While humans typically have 200,000 light-sensitive cells per square millimeter of retina, eagles may have 1 million – five times more. Similarly, while humans have only one fovea, a funnel-shaped part of the retina where vision is sharpest, eagles have two. Finally, where people see just three basic colors, eagles see five, enabling them to pick out even well-camouflaged prey"
However this some how doesn’t quite answer my question completely that being: Is eagle vision stereoscopic?
The Britannica says that Eagles are of the “… family Accipitridae (order Falconiformes)” and says of the falconiformes:
“The eyes are globose and move little in their sockets. To see behind it, a falconiform must rotate its head. The focal length is adjustable by muscles controlling the curvature of the lens. Forward binocular vision is through 35–50° of arc. The proverbially high resolving power of hawks’ eyes depends partly on a large image on the retina, partly on the concentration of rods and cones. There are two foveae (areas of high visual acuity), one laterally directed for monocular vision, the other forward, for binocular vision; in each fovea the visual cells are still more concentrated, providing resolving power up to eight times that of the human eye. There is a screenlike pecten, which may cast a lattice-like shadow on the retina, permitting good perception of shape of moving objects. The iris in many adults is yellow, red, or orange.”
Eagles have slightly stereoscopic vision, like most birds. There is some degree of overlap between the fields of vision, but it’s not as great as in humans or owls. This is made worse because the eyes can’t swivel around to focus on a specific object. I’m not sure how well their brains are able to translate this overlap into 3d images.
However it doesn’t matter much. Eagles mostly need to see things a long way away. Like you said, a few miles. Even in animals with stereoscopic vision it’s useless as a rangefinder at this distance. The triangulation length is relative to the distance of the base, and with eyes a few inches apart it doesn’t need distance much for the system to be useless. In fact anything over about 40’ is too far for human eyes to judge distance using binocular vision. After that we just use relative sizes, which is exactly what eagles would use.
Ever see those optical illusions with with the whacky room? You know the tiny room with the oddly sloping walls where as the person goes back a few feet it looks like the have travelled a dozen yards? The reason this works is because at range we judge distance by relative size and perspective, not by using binocular vision. Place a small man next to a tiny horse under a low roof and in the absence of other visuals clues we think he’s big if we are more than 40’ away. Closer than that we can judge size and distance directly
I’ve read that chameleons can determine distance on the basis of focal length (their eyes change shape to focus on an object, and so by knowing how much their eyes change shape they know what the distance is), but I don’t whether that’s true. Actually, humans can probably do it to some extent.
That isn’t particularly true. Predatory birds usually have about the same degree of binocular vision as herbivores. Ditto for mammals, reptiles and fish.