Our word migration comes from the Latin word migrare meaning to wander. Our word hibernate comes from several Latin word: hiems meaning winter; hiberna meaning winter quarters; himalays meaning winterland (Himalaya Mountains) and hibernare meaning to pass winter.
With our modern technology, we now know the only bird that is known to hibernate is the poorwill, Phalenoptilus nuttalli. The genus name comes from two Greek words: phalaina meaning moth and ptilon meaning feather or moth-feathered in reference to the soft moth-like texture of the plumage. The species name was given by John Audubon in 1844 in honor of Thomas Nuttall, who after coming to America became a distinguished ornithologist.
The common name of poorwill comes from the bird’s cry. The Common Poorwill, is a member of the nightjar family of the arid west. The Common Poorwill is considered the western counterpart of the related and similar eastern Whip-Poor-Will, and in the past, these two were considered a single species.
The Common Poorwill could enter hibernation in response to environmental stress, such as lack of food and inclement weather. Reports from California and New Mexico show that populations occurring on the southern edge of this species range, in the United States, enter into hibernation. In 1804, Meriwether Lewis reported finding these hibernating birds as far north as North Dakota. The northern populations typically migrate south to winter in warmer climates rather than hibernating.
The Common Poorwill, which can slow its metabolic rate and drop its body temperature, goes into a hibernation-like state known as "torpor." In periods of cold weather, a poorwill could stay in torpor for several weeks. Although some consider this not a true hibernation, torpor allows the bird to go long periods of time without food. Torpor is also an aid in surviving cold spells when insects, which make up their diet, are not active. In a laboratory, Common Poorwills readily enter torpor when deprived of food.
The Common Poorwills are mostly migratory birds that are native to Canada, United States and Mexico. They summer throughout the western half of the United States and Canada and migrate south to winter in the extreme southwestern United States and Mexico. In Southwestern United States, they could occupy the same region year round but migrate to higher elevations for the breeding season and then to lower elevations during the winter.
Other bird species go into a temporary hibernation that is referred to as torpor, which is not a true hibernation and only lasts for shorter periods. Birds such as chickadees, kinglets and titmice generally go into this torpor on cold winter nights to conserve energy. To come out of this torpor state, the bird begins to shiver, which raises its body temperature and uses up body fat. The body fat of some chickadees when measured in early evening was seven percent and only three percent when measured in early morning. To replace this fat, birds must continually feed throughout the day. If not enough body fat can be obtained, the bird will die on the next cold night.