The Common Poorwill, noted for its distinctive call and its ability to enter a state of deep daily torpor under natural and laboratory conditions, is the only bird known to spend long periods during winter completely inactive. Common Poorwills are perceived to be rare or uncommon residents throughout the high rolling prairies, semi-arid flats, and rocky foothills of western North America, a region that forms its chief habitat. Because this is a nocturnal, cryptic bird, however, it can be missed in surveys, and recent studies have shown it to be more abundant in many areas than was formerly appreciated.
The Hopi Indians refer to the poorwill as Hölchoko, "the sleeping one," and the Pima Indians also knew about the bird's ability to use torpor. Studies of individual birds monitored using radio telemetry have shown that under natural conditions daily torpor is used regularly by this species, often but not strictly outside the nesting season, and apparently in response to low ambient temperature and/or reduced food availability. Recent work has shown that individuals in the southern part of the range remain completely inactive on as many as 90% of all winter days. Body temperatures as low as 5°C have been recorded in torpid birds, with oxygen consumption reduced by over 90%. In addition, laboratory studies have shown that these birds tolerate with remarkable efficiency the extreme heat of their prairie and desert environments; panting, gular flutter, and cutaneous water loss all contribute to the efficiency of their evaporative cooling system.
Like other nightjars (Caprimulgidae), this species feeds primarily on flying insects, including moths and beetles, generally caught as the bird sallies out quickly from a low perch or the ground. This is a ground-nesting species, laying two eggs in a slight hollow scraped in the bare earth; both parents incubate eggs and brood young, which are fed regurgitated insects. Breeders are purported to move their eggs and are known to move young short distances in response to disturbance.
Much of the basic biology of the Common Poorwill remains elusive, however, owing primarily to its nocturnal activity and cryptic plumage and behavior. Little is known about its behavior and physiology under natural conditions, its habitat requirements, migration, or winter distribution, and the biology of the species has been studied mostly at the northern limits of its range. The species is divided into 5 subspecies based on geographic distribution and plumage color.