When colder winter weather arrives I get out my winter coat. It is heavier and warmer than my fall jacket. And despite much colder outdoor temperatures it keeps me warm.
Birds do the same thing.
One ornithologist counted the number of feathers on common house sparrows in mid-summer (July). The average total was 3,150-3,200 feathers. Then he repeated his count again in mid-winter (Jan/Feb). In the colder temperatures of Jan/Feb the average number increased to 3,550-3,600 feathers, about 12% more feathers in winter than in summer. Most of the additional feathers were down feathers and, if you ever had a down jacket or used a down sleeping bag, you understand how down retains warmth.
Sometimes on a blustery winter day you will notice a chickadee that seems puffed up, much more plump than normal. The chickadee has fluffed its feathers trapping more air between the feathers thus creating added insulation. A chickadee’s normal body temperature is around 104 degrees. When the outside temperature drops way below freezing all that separates its body from the frigid outdoor cold is ¼ inch thickness of feathers.
It seems very practical for birds to grow additional feathers in winter to provide greater insulation.
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