The Golden-crowned Kinglet weighs six grams, about the same as two pennies, yet winters as far north as Alaska and Nova Scotia. The birds move through the forest in small flocks and feed constantly, taking in enough tiny caterpillars to maintain their internal furnace at 110°F. And their insulation keeps them warm. Their feathers make up 8% of their body weight, equivalent to the weight of the clothing of an arctic explorer.

More info and transcript at BirdNote.org. 

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