‘Eagle eyes’ vs. ‘See like a hawk‘
So which one is it? Which bird has the best eyesight, and which statement is technically the most accurate way to describe someone with great vision?
Before we answer this question, let's quickly touch on what having great vision actually means. For humans it typically refers to a person’s visual acuity, which is a measure of the clarity or sharpness of vision and typically tested using the Snellen chart. A Snellen equivalent visual acuity of 20/20 means a person can see at 20 feet what a person with "normal" vision can see at 20 feet. For humans, excellent visual acuity is important, but for some animals being able discern subtle motion is more important. While both scenarios involve exceptional vision, for our discussion lets use Snellen acuity as our metric of vision.
Now that we have defined our goal, we need also describe the scenario. There is a wide variety of lighting conditions that different animals have adapted their vision to with some animals having exceptional high contrast, daytime vision and others with superior vision at night or in low contrast conditions. Since humans have significantly better daytime vision than nighttime, and we are attempting to choose which descriptive term best describes a human with excellent vision, lets add daytime lighting conditions to our question.
So without further ado, which bird, an eagle or a hawk has the best visual acuity as measured by Snellen acuity during high contrast daytime conditions.
Drum roll…… the winner is the eagle!
The size an eyechart letter would need to be for a human with 20/20 vision to be able to discern what letter it is at 4.7km away. The smaller E is the size an eagle would need it to be.
When it comes to razor-sharp vision, eagles take the crown. Their vision is estimated to be 3.6 to 3.8 times better than that of humans. This translates to a mind-boggling Snellen equivalent visual acuity of around 20/5! Imagine being able to see at 20 feet what the average person can only discern at 5 feet. Imagine standing on a beach and gazing out at the horizon, which would be about 4.7km away. A human with 20/20 vision would be able to see and discern a Snellen Eyechart letter if it was at least 6.8m tall. An eagle would be able to see this same letter easily at this size, and still be able to see it if we shrink it down all the way to being 1.7m tall.
The 6.8m tall E on the container ship is barely discernable at this distance, while the smaller 1.7m E is all but invisible to humans.
Hawks are no slouches in the visual department either. Their keen eyesight is approximately 2.6 to 2.8 times sharper than ours, boasting a Snellen equivalent visual acuity of around 20/8. These raptors are designed for precision hunting, allowing them to spot their prey from great distances with astonishing accuracy.
Another bird that deserves mention in this conversation is the owl. While owls may not have the same daytime visual acuity as eagles or hawks, they possess a different superpower: exceptional night vision. Their eyes are adapted for low-light conditions, and while it's challenging to quantify their visual acuity using Snellen charts, it's safe to say that in the dark, they reign supreme. Some studies suggest an owl’s vision in the deepest dark of the night is equivalent to a human’s best daytime vision.
Finally, diving into the underwater realm, we find a crustacean that also deserves honorable mention: the mantis shrimp. These colorful critters have compound eyes that allow them to detect an extensive range of colors, including ultraviolet light. While Snellen charts don't apply here, their ability to see a world beyond our human perception is mind-boggling.
From eagles with their telescopic eyes to mantis shrimp unlocking a world of color, the animal kingdom is a visual wonderland. While we humans hold our own in the visual acuity arena, it's humbling to realize just how diverse and extraordinary vision can be in the animal world. 🦅👀