What does it mean when scientists say “an eagle can see a rabbit in a field from a mile away”. Is their vision automatically more zoomed in? Do they have better than 20/20 vision? Is their vision just clearer?
In: Biology
They have 20/5 vision. Meaning that they can see things as clearly at 20 feet as you can see them at 5.
Bird eyes tend to be better than those of mammals. More detector cells in any given surface area* (at the spot where we can focus best we have the equivalent of a 56 megapixel camera. For an eagle, where the largest eagles have eyes as large as ours, this resolution is about 200 megapixels), larger eyes compared to the size of their head (so ostriches have the largest eyes of any land animal), no bloodvessels on the inside of the retina, They also have multiple muscle rings that helps the lens in your eye to focus (which helps with viewing things very close up or very far away) plus a scleral ring (a ring made from cartilege and/or bone) that helps support the eye and the muscles that helps the eye focus.
The drawback to bird vision is that their eyeballs can’t move. Many species don’t have the muscles for it (and the scleral ring inhibits movement on the birds that do), so they need to move their heads to focus on something (which leads to the weird headmovements you see in for example pidgeons and chickens).
*And a 4th colour receptor cell that allows them to see intro the ultraviolet spectrum.
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