24 January 2014

Friday Field Notes: Icy Sentinels: Stalwart Herons in Winter

Riding my bike in the summer along the Clark Fork River sometimes feels like an exhibition of nature. Munching beavers, darting kingfishers, and watchful Osprey are rarely out of sight or unheard. Trout break the surface of the river and grebes and their allies dip to catch a morsel under the cool water. But, as the warm clear mornings start to turn into cold ones and ice forms along the river's bank, only the hardiest animals show themselves.

A watchful eye in winter will turn up some of these unyielding creatures. One not too hard to spot is the Great Blue Heron. I was amazed to see that these large graceful birds were waiting at the edge of the ice, staring motionless into the water, as if mesmerized by the chunks of ice floating by. My thoughts then turned to a poem by renowned poet Mary Oliver--Herons in Winter in the Frozen Marsh.


This poem captured much of the feeling I experience while watching these icy sentinels.

If you look carefully at some places on the river ice you can see their "pitchfork" footprints along the water's edge. You may be wondering how the herons can stand the cold with their long scaly legs in the frigid water or on the ice, and the answer is that they are equipped with a special routing of blood vessels in which the vessel carrying blood out to the leg lies directly against the vessel carrying blood back in. This is called "counter-current heat exchange," and is an adaptation used by many organisms across the globe. The blood going into the legs warms up the blood coming back into the body, keeping the heron's body temperature at a remarkable 103 to 106 degrees Fahrenheit (keep in mind that the average human body temperature is 98-99 degrees Fahrenheit). This allows them to stand at or in the water's edge and watch for fish and crustaceans for long periods of time.

As for me, I feel fortunate that I don't have to stand barefoot in the cold river to catch a bite to eat. I'm content just watching the pros at work.

So next time you find yourself along the river, make sure to scan for herons, motionless and cold, looking for lunch.

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