Small and warm, hummingbirds are about at the size limits for a terrestrial closed circulation. The smallest hummingbird, the Cuban bee hummingbird, is about six centimeters long and weighs less than three grams. Ruby throated hummingbirds weigh about three grams and are about nine centimeters long having resting body temperatures around one hundred and five degrees F when beating their wings forty to eighty times a second as they burn large quantities of energy in rapidly contracting fast red muscles. A Ruby's resting heart rate is around two hundred and fifty beats a minute, and it breathes at about the same rate. But a Ruby's heart rate rises to twelve hundred beats a minute when feeding on the wing. How does the little heart do it? (Ref: Hummingbird and Shrew Energetics).

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