to Escape the Great Blue Heron

 

 Please click the image to enlarage. _RLK2023 crop 

 

I captured this image in 2015. It is now May 20, 2016 in the 47th north latitude and the bullfrogs have still not emerged on this pond of my boyhood. It seems late to me, but I am sure they will begin calling within 1 week. That is when I will locate the first one of 2016.   Just checked last year’s dates. This above image was captured June 16, 2015. But the first bullfrog I photographed last year was on May 29, 2015.  They are slightly overdue.

The aquatic vegetation is just beginning to emerge (May 14, 2016) and last winter was an exceptionally mild one, both in terms of snowfall and low temps. Most winters here on coastal Maine are much more severe than last year’s. Of course, there might be no correlation between the two. But “ice-out” in southern Maine was very early this spring (late March). It usually occurs no earlier than mid-April and most often very late April.

the AMERICAN BULLFROG’S PRECARIOUS EXISTENCE: The following is a short explanation for why the American Bullfrog really does live a precarious existence. 

The spring-emergence of bullfrogs is quite closely synchronized with the emergence of aquatic plants. And the reason for this is that the frog must have the vegetation for escape cover. Here is why: In the Food Web, American Bullfrogs are prey to the Great Blue Heron (GBH), both NA Bass species (Mycropterus spp.) and less occasionally the Snapping Turtle. But overwhelmingly, its greatest danger is the GBH. A bullfrog in the open is easy prey for the ever-present GBH. The GBH begins hunting at first good light and probably continues hunting all day, or until its hunger is satiated. On this pond, there are 2-3 great blue herons that hunt every day.

At best, the American Bull Frog has a poor escape mechanism.  It is interesting that this is the same escape mechanism the American Alligator uses in the Everglades.  I have watched gators submerge to a very shallow bottom that I can clearly see.  They remain motionless there, with their eyes closed, unaware that I am watching them from above.

There are many hunting strategies that predators use.  But most predators specialize in one hunting strategy.  Some ambush, some actively search and some take a position that allows them to watch an area, such the surface of a water body.  

Arguably, the long-legged wading birds are some of the most effective hunters of all aquatic macrofauna.   And the Great Blue Heron and Great Egret (of more southern climes), are perhaps the most efficient predators of them all.

The GBH uses a combination of a very slow stalk and then wait.  And it is very patient.  The GBH is built perfectly for the habitat it hunts and it hunts the habitat that is very species-rich.

So let’s take a closer look at how the GBH is such an effective predator:  

Its long toes keep it buoyant on the soft and deep detritus that is most often present within habitat the GBH hunts. The GBH’s long legs and long body and neck, give their eyes a very lofty perspective of any fauna activity occurring in the shallows.  From here, with its head absoluteluy motionless except when it is moving forward to its next still position, the GBH can visually detect the slightest of movement in the vegetation below.  That is because there is no wind where it hunts and so the submergent vegetation is not going to move…. unless a submerged animal moves it. And the GBH will spend most of its hunting-time standing absolutely still, with no head movement as it intensely scrutinizes the shallow vegetation for any movements.

To any aquatic fauna that is looking upward, the GBH’s plumage is cryptically colored against similarly colored foliage along the water’s edge. And this is because of a phenomenon called positive phototropism: Plants along the edge of any habitat transition, grow away from other competing plants, as they seek sunlight. So the trees and bushes along the edge of any aquatic system ALWAYS grow out toward the open water, bending out and overhanging the edge of the water. And since the GBH is hunting the edges, it is most often under the trees’ overhanging branches. And this is the reason for the colors of GBH’s plumage. Add to all of this that the shallow edges of a body of water are the most productive. The dense, deep organic material supports the entire base of the aquatic food web. This littoral zone also serves as the most productive nursery area of the entire freshwater basin. The GBH hunts this habitat because this is where the highest diversity of species and biomass of prey are found. In short, bullfrogs are never found in sandy, sparsely vegetated habitat.

Even within the cover of aquatic vegetation, I have always found that any frog is easy to spot and catch-by-hand after it has just submerged. The bullfrog lives in very shallow water, immediately adjacent to the shoreline.  Of course it would never attempt escape onto the shoreline. And it cannot go out beyond cover of the aquatic vegetation in the littoral zone, or it is instant, easy food for large bass.

When escaping perceived danger, bullfrogs go directly to the bottom, not moving too far laterally. On the bottom, they shut their eyes and simply remain absolutely motionless.  So far, this sounds like an effective way to escape the GBH doesn’t it?  But there are at least two reasons the frog is very vulnerable.  First, with its eyes closed it is oblivious to anything around it.  Second, if the GBH saw the bullfrog as it is submerging, the frog has no chance for survival.  So even though the bullfrog may now feel secure as it remains motionless in the surrounding vegetation, the Great Blue Heron knows there is a bullfrog down there and it is now actively searching for it.   

It is important to note here that birds have color vision.  Excepting the primates, mammals do not have color vision, but birds to.  So, even though the bullfrog is motionless, the GBH has a chance off differentiating it from the vegetation it is hiding in.  And oftentimes, the bullfrog will make just one slight movement, as it readjusts its position.  That is its undoing.  The GBH often rivets its attention on one spot, slowly lowers its head and closely scrutinizes.  When a GBH does this, a strike is imminent.   It may miss a fish, but it will not miss a bullfrog.

GBHwithRed-BreastSunfish

The GBH will kill and eat any animal that it is capable of subduing and even some that it finds it cannot swallow. Among wading birds, the Great Blue Heron is the most assailing. There’s an abundance of documentations of the GBH attacking and trying to swallow mammals that are just too large to swallow; i.e. squirrels, rabbits, muskrats…to name a few. I once knew a GBH that made its living in upland fields by spearing/swallowing Boreal Red-backed Voles. Bullfrogs are solitary and territorial, each laying claim to a specific piece of shallow water, along the shoreline. And the shallows, along the pond edge, are the hunting grounds the GBH favors.  

 

 

 

 

 

 

 

 

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