-- HUMMINGBIRD Q & A --
Physiology #1

1. Do hummingbirds hibernate?

• Animals have several ways of dealing with cold weather. Some put on heavy winter coats, some hibernate all winter, and others migrate to warmer climates where food is more available. Ruby-throated Hummingbirds (Archilochus colubris) are migrants and do not hibernate. However, in cool weather they do undergo an overnight metabolic change called "torpor." On cool nights, the body temperature of hummingbirds can drop from a daytime norm of about 40.5 degrees C (105 F) to an overnight low of about 21 degrees C (70 F). Torpor also allows the heart and breathing rate to slow and lowers the basal metabolism so that the hummingbird burns much less energy overnight. On some cool mornings in late spring and early autumn, people encounter RTHUs that are sitting motionless and appear to be dead, asleep, or "hibernating." These birds are probably in torpor and should not be disturbed. As the day warms their metabolisms will speed back up and they'll get back to doing what hummingbirds do.

2. My hummingbird appear to get bigger in late summer. Do they gain weight before migrating?

• Ruby-throated Hummingbirds (Archilochus colubris), like many migrant birds, go through hyperphagia prior to migration; i.e., they eat almost continuously, lapping up carbohydrate-laden nectar and sugar water while consuming prodigious numbers of tiny insects that are full of fats and proteins. They use up the sugar almost instantly during everyday activities--including hyperphagic frenzies-- but apply the protein toward strengthening their all-important flight muscles. At the same time, they store surprising amounts of energy-rich yellow fat that will burn off efficiently during migration. This fat is laid down primarily in the furcular region (wishbone) as illustrated at right, but also forms at other locations of the body--including along the flanks where the legs attach to the body. These fat deposits tend to stretch the skin and probably cause the feathers stand up a bit, making the hummingbird look even fatter than it already is. Interestingly, a 3.0g male RTHU may put on 2-2.5g of fat during hyperphagia--nearly doubling its mass before migrating across the Gulf of Mexico to the Yucatan! (Thanks to Lynn Ketcham from Houston, Texas for submitting this question.)

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