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Male Anna's
Birds' feet have a miraculous adaptation that keeps them from freezing. Rete mirabile — Latin for "wonderful net" — is a fine, netlike pattern of arteries that interweaves blood from a bird's heart with the veins carrying cold blood from its feet and legs. The system cools the blood so the little blood that goes down to the feet is already cold, so the birds don't lose much heat. The small amount that goes to the feet is likely just enough to keep the feet from freezing.
Hummingbirds enter torpor (lowered breathing and heart rate) to conserve energy. This usually happens on cold nights, but sometimes they go into a torpid state during the day. When they're sitting, their feet automatically clamp down, but on a smooth perch, they may slip and wind up hanging upside down.
Edited on Dec 15, 2023 at 01:03 PM · View previous versions
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