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A couple more factors that influence depth perception. The distance between the eyes must influence the perceived depth. When the eyes are very close together, their effectiveness at depth perception is probably limited. It would be interesting to find out how this plays out in the animal world. How close can eyes be and still deliver binocular depth perception? Binocular depth depends on the difference between the images falling on the right and left eyes, and when these are very close together, the image differences would be small. The second factor is that distance (depth) can be estimated with a single eye by parallax, and a lot of birds (e.g. doves, chickens) seem to do that by moving their heads back and forth, especially while walking. Close objects then appear to move across a larger segment of the visual field than distant objects. It's easy to confirm this effect for oneself. Parallax might be more important for birds that have eyes on the sides of their head, and/or that move faster.

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Really interesting, Walter - thank you! I looked up parallax, and in addition to being a restaurant that serves chicken in Cleveland, Ohio (my first search terms were not specific enough) I found this site that describes how chickens (and pigeons) track objects by bobbing their heads back and forth: https://brainracker.wordpress.com/tag/motion-parallax-2/

It is also, apparently, why your cat will bob its head back and forth before making an ambitious jump. Thanks for introducing this term to me!

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I found out about parallax when I wrote about a cute little bird called the tūturuatu. They're so adorable, and incredibly endangered.

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I just looked them up - there are only 250 of them left!

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Yes, and there's only one population which is relatively secure. With most of our endangered birds we try and get populations on several different islands so there are multiple populations, but attempts to establish these birds in other locations have failed.

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If you're interested, here's the article I wrote about them a couple of years ago. https://theturnstone.substack.com/p/distress-call

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Thank you! I was going to ask if you could link it

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Aug 27Liked by Heather Wall

I am late to this party but stumbled upon this article while looking up the same question. I looked it up because we have a resident male Great Blue Heron here in a nature reserve that I photograph often nearby and he thrives with just one eye. He does have to turn his head completely to look around when other birds or animals approach from the missing eye side but, he fishes, flies and does really well and has had a missing eye for years.

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Fascinating! It was while watching a Heron fish that this question occurred to me - so glad your heron has adapted well!

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May 3, 2023Liked by Heather Wall

When I was diving off the coast of Venezuela, the guide told us that the sea gulls slam down into the water on one side of their body , especially eyes, causing eventual blindness and that they could no longer see to accurately catch fish and eventually fly away and die of starvation. After reading your article, I wonder if his information was true.

Laura L.

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Interesting! I hadn't heard that about sea gulls, so I did a quick bit of research and found that it's apparently something tour guides tell their customers, usually about pelicans though I did find some about gulls as well. It looks like it's probably a myth since pelicans have an air sac in their bodies built to absorb the impact as they crash into the water. https://www.allatsea.net/brown-pelicans-myths-and-facts-part-i/

Not sure about gulls. But either way, it seems to reinforce the idea that they need two eyes to survive...

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