We've all heard about mammals hibernating, sleeping through cold weather while dropping their body temperature to just above freezing. And most people realize that lizards and snakes disappear during the cold season, lying asleep under ground or in secure dens. Few of us, however, visualize those exemplars of hot-bloodedness, birds, as being anything other than—well, hot blooded.
Nonetheless, both short-term and long-term decreases in metabolic rates
are known among birds. Some hummingbirds slow down radically during cold nights. At 95
degrees, the heart of the Blue-throated Hummingbird, for example, normally beats
between about 480 and 1260 beats per minute, depending on activity; when torpid at 59
degrees, heart rate drops to about 36 per minute. The alternative is to burn fuel at a
frantic rate to keep functioning as normal—fuel that may not be available. A bit
closer to home, in the Chihuahuan Desert, the Poor-will has been known to drop its
temperature to as low as 43 degrees and to remain dormant for up to 88 days during cold
weather. Hot blooded? Not always! 
Contributor: Arthur H. Harris, Laboratory for Environmental Biology, Centennial Museum, University of Texas at El Paso.
Desert Diary is a joint production of the Centennial Museum and KTEP National Public Radio, University of Texas at El Paso.
Welty, J. L. 1982. The Life of Birds. 3rd ed., Saunders College Publishing, Phildelphia, 754 pp. (See pp. 114-115, 153)