Thursday, November 21, 2013

Great Blue Heron

I found this Great Blue Heron in almost the only open water I encountered on 19 November 2013. Air temperatures were slightly below freezing and most of the local lakes sported a thin coat of ice. (The relatively warm weather of the last two days has transformed the lakes back to a liquid state, but the bitter cold predicted for the weekend should reverse the ice-free trend.)

When they are cold, herons droop their wings to absorb warmth from the sun (although this bird stood in the shade); they also avoid wind by seeking shelter, in this case behind a steep riverbank. Note that the heron is standing on one leg, keeping the other tucked up under its ruffled feathers. Presumably the heron is also using a counter-current exchange in the wet leg. This system exchanges the warmth of arterial blood with the cold venous blood coming from the lower foot. The result is that most of the bird’s heat is retained near the body rather than lost to the cold water.

Stragglers are known to attempt to winter in Minnesota, but most Great Blue Herons winter south of the state. The spots on the wing coverts and the solid-gray crown indicate this bird is in its first year. This heron did not appear to be wounded or sick (although it walked away rather than flying).

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