What’s the last thing a fish sees…

before it’s snatched up by a heron?

Great Blue Heron.
Great Egret peering into the water.

The heron in the above photo is actually looking down at me, not a fish, but you get the idea. The heron was in a pine tree above the path in Explore the Wild.

What’s interesting about the photo is that it clearly illustrates the fact that herons have binocular vision and can see objects beyond and below their bills, helpful if you make your living plucking fish out of the water. Among land or air based predators binocular vision is the norm.

While herons’ eyes are placed on the sides of their heads allowing them to see potential trouble heading their way from the side, they are far enough forward that both eyes can be used to peer down into the water at their prey. Two eyes are better than one when trying to gauge position and distance.

Herons are able to detect trouble approaching from the sides as well as watch for prey below.

Birds such as mallards, and mammals like rabbits, have their eyes placed high and laterally dead center on their heads allowing them a 360 degree view of the world. It’s tough to sneak up on either. Although herons can’t see directly behind themselves, a simple turn of that long flexible neck lets them quickly checkout who’s sneaking up from behind. They give up some of their ability to see potential predators approaching from the rear in order to have bino vision, but you still have be be on your tippy toes to sneak up on them.

OK, so a heron can see with binocular vision helping it gauge the distance and location of the object of its attention (fish in water), but how does it deal with refraction? You know, when you put a stick into the water it looks like it’s bent. The light is bent by the different (relative to the air) refractive properties of the water. Unless the fish is on the surface, say it’s 6 inches or so under the water, it isn’t actually where it looks like it is. How does the heron allow for refraction when jabbing at a fish in water?

I’ll leave that one to you.

8 responses to What’s the last thing a fish sees…

  1. FIRDA says:

    Sounds like we should see the harrying while it lasts. I’ve seen it twice and it’s impressive! GBH eyes are great – you must have been close to “splat” range to get that shot. Way to go!

  2. julie says:

    Sounds like we should see the harrying while it lasts. I’ve seen it twice and it’s impressive! GBH eyes are great – you must have been close to “splat” range to get that shot. Way to go!

    • Greg Dodge says:

      Heck yeah, splat range for sure. But hey, nobody said it’d be easy. There’re certain risks you just gotta take!

  3. Wendy says:

    That top picture is an award-winner, for sure! Looks like the eyes are dilated differently. Is that normal?

    I’d guess that the heron allows for refraction in the water by attacking his prey with an open beak.

    • Greg Dodge says:

      Yes, I first noticed the different pupil dilation in the eyes of birds as a bird bander. You get real close looks when you have a bird in the hand. While holding one such bird in hand I noticed that the pupil of one of the bird’s eyes, in sunshine, was constricted while the pupil in the other eye, in shadow, was almost wide open, dilated. As the bird moved its head the light hitting the eyes changed and the pupils reacted to the changing light independently, and very quickly.

      I thought that this ability of the bird’s pupils to react independently to changes in light was unique to birds, and maybe it is, but I think that it may actually be present in other animals as well, certainly fish. After all, in most fish, as in many birds, the eyes are often positioned on completely different sides of the head. The eyes are always looking at two separate scenes; one could be brightly lit while the other is in deep shadow.

      Do people’s eyes dilate independently?

      I just did an experiment using Bobbi Jo (Butterfly House) as my Guinea Pig. I asked her to cover one of her eyes with her hand to block out the light. When she covered one eye, the pupil in the other eye dilated, meaning we, humans, do NOT posses eyes that react independently to light changes, both pupils dilate and constrict together.

      I think that the pupils’ ability to react independently to light in birds is an adaptation to their lifestyle. Flying in and out of shadow, as through the woods, around objects, and just trying to make a living in a high contrast world would definitely be aided by eyes that can handle light in the way that a bird’s eye can handle it, with eyes that can act independently of one another.

      By the way, most birds’ eyes can be moved independently as well, unlike us humans who, when we move one eye the other one goes with it. Owls however, cannot move each eye independently. In fact, the eyes don’t move at all, but their head can spin around about 270 degrees.

      As far as the refraction problem with catching fish, herons have had millions of years to evolve into what they are. They probably have some sort of mechanism built into their make-up which allows them to compensate for the refraction. What that mechanism is is the question. The ones that don’t have this ability don’t catch fish, and probably don’t live long enough to create more herons.

      Herons do indeed jab at fish with an open bill. They do not spear them as is sometimes thought.


      • troy becker says:

        It amazes me that these herons can catch the fish in water and also catch voles etc on land, which indicates to me that they can know when to account for refraction and when not, and presumably how much. Can anyone link to any studies on this? It’s fascinating. I saw (google) that cattle egrets cannot seem to correct for refraction, and this book: Vision Brain and Behavior in Birds (edited by Ziegler and Bischoff). Any summations or findings?

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