How vultures see so far
How vultures see so far
Some fun facts and science about how vultures see
Last week I was out with some friends and someone who had just come back from a safari said, ‘I heard vultures can see prey from 100km away, how is that possible?’ I gave a simple answer about how a vultures eye must have a pupil that is bigger to more sensitive to light since eyes can see any distance (the problem is when things are in the way, blocking the light). Someone laughed and said ‘How can you possibly know that?’ Implying quite heavily that I was talking out of my ass. Well I decided to see if I was right. Because frankly, I was talking out of my ass, but it was an educated ass thought.
How is it possible that vultures can see prey 100km away?
The first thing to do is to validate the assumptions in the question. First, does a vulture have the opportunity to see for 100km over the earth before meeting the horizon (the earth isn’t flat remember)? And second, is it in fact true that vultures can see something from 100km? As I mentioned in the intro, we can all see over 100kms away (the moon is 384,400 kms away and you can see that) the question is whether vultures can see prey that is 100km away through the air. Then we can talk about how. Okay.
Does a vulture have the opportunity to see over 100km away?
Yes. According to BBC science focus, ‘if you assume very flat terrain and good visibility, you can multiply your height in metres above the ground by 13, and take the square root of that. This is the distance to the horizon, in kilometers’. And according to the Smithsonian institute ‘Rüppell's griffon vultures have been documented as having reached heights of over 10,973 meters above sea level.’ This is the highest flying species of vulture I could find so we’re going to use it for reference since I don’t know what species my friend heard about on safari but he was in Zanzibar and the Rüppell's griffon vulture has been known to fly that far south from it’s native Kenya, so it’s possible.
If we assume that we’re talking about a Rüppell's griffon vulture flying at 10,000 meters above sea level which they have been known to do (nicely in the troposphere) we can calculate that the horizon is 360km away. So our vulture doesn’t need to fly that high to see 100km away. Some quick maths tells us that if the vulture instead flies 770m high, rather than 10,000m, the horizon is just more than 100km away. So let’s say for the rest of this that the vulture is flying around 800m above sea level.
For reference, since height is hard to imagine at these scales, commercial planes fly between 8,000 - 11,000 meters high. Much higher than our vulture.
Can vultures see something from 100km away?
Your a vulture, you’re flying 800ms up, can you see a lovely tasty carcass close to the horizon? If we assume a sizeable carcass, around a meter square, the answer is … no. Damn. I was wrong.
I couldn’t find any hard research done specifically on vultures, never mind the species we’re talking about but it is widely written that a ‘Black Vulture’, native to the US, can see prey up to 4 miles (~6.5km) away. This isn’t substantiated by any research I could find in my 15 minutes of trying, but it’s nowhere close to 100 km away.
So while the Kenyan Rüppell's griffon vulture can fly twice as high as the US Black Vulture, even if we double the range of seeing prey (which probably isn’t proportional), it can’t see anything so small as a meter square that far away. You see folks I forgot about the need to focus all of that light. Vultures would need to have very odd eyes to see details that far. Think about how telescopes do it.
How can vultures see so far?
Well, just because I was wrong and vultures can’t see a carcass 100km away doesn’t mean we can’t talk about how their eyes work, they can still see things very far away. How do they do that? Well, I’m glad you asked.
Eyes are pretty well understood things. Light passes through the cornea (the clear front layer of the eye) which is shaped to bend and focus the light into the right place. The amount of light that passes through is determined by the size of the iris which opens and closes the pupil (your eye sphincter).
Next, light passes through the lens (a clear inner part of the eye) which works with the cornea to focus light correctly. It focuses onto the retina, a light-sensitive layer of tissue with photoreceptors or visual receptors that turn light into electrical signals for the brain to deduce.
Research published almost 10 years ago in the American association for anatomy that compares the eyes of two American species of vulture nicely describes the components of the eye that exemplify a vultures vision. The study was to understand the difference between a lower flying vulture that was theorised to predominantly use it’s sense of smell for finding food, and the Black Vulture, that flies much higher.
The authors of the study did basic measurements of the eyes of dead birds measuring lens diameter, the shape of the entire eye, and the ratio of the weight of the eye in relation to overall body mass. Then they examined the density of the visual receptors on the retina. A retina with a low density of visual receptors indicates more poor vision.
The study concludes that both species of vulture have similar eye morphology and both have densely packed visual receptors and so have roughly equivalent vision. But the reason they can see so much further than humans is the densely packed visual receptors.
These high magnification digital photos from the study show the high density of photoreceptor cells in the central region of the retina in the eye of a turkey vulture, left, and the eye of a black vulture, right. The high density of receptor cells in this region indicates both birds have keen eyesight.
Conclusion
I’m not sure why I wrote this one but I learnt that certain species of vulture can fly very high, can see very far, and can see further than humans because their retina is packed with a more dense covering of visual receptors and so are more sensitive to light. However they can’t see a carcass 100km away. That’s too far. Our best guess is they can do about 7km, which is still pretty far.
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Great research and information.