All animals need energy to survive, and they get this energy from the food they eat. When food is limited, animals need to conserve their energy until food is readily available again. One way to conserve energy is to exert less throughout the day and/or night. Because all major life functions (breathing, moving, pumping blood, maintaining body temperature, etc) require energy, reducing body temperature or slowing heart rate is one way to conserve energy. Many animals do this during hibernation, but other animals, such as birds, do this for shorter periods of time (anywhere from 24 hours to a few days), which is known as torpor.
Many birds enter torpor when it is difficult to hunt for prey and find food, for example during moonless and/or cloudy nights, when ambient light levels are low. The birds that most commonly engage in torpor are small-bodied specialized foragers, such as hummingbirds. Although there is considerable variation in patterns of torpor among birds, one species, the common poorwill (Phalaenoptilus nuttallii), is the only bird known to remain torpid for extended periods of time. Poorwills feast mainly on insects, and experience substantial seasonal fluctuations in food availability, and thus are much less active in the winter. To cope with the decrease in energy availability, common poorwills reduce their body temperature during winter, entering a hibernation-like state. This ability to engage in prolonged periods of torpor allows the common poorwill to conserve energy during times of limited food availability so it can forage with an increased likelihood of success.Edit Summary
Common Poorwills, small nocturnal insectivorous birds found across western North America, are seemingly unique because of their alleged ability to remain torpid for extended periods during winter. We used temperaturesensitive radio transmitters to assess patterns of torpor use at sites in the Sonoran desert of southern Arizona. Poorwills used torpor extensively whenever ambient temperature (Ta) dropped below 10° C, and there was little evidence for thermoregulation when Ta was above 5° C. During the winter months (December through February), birds remained entirely inactive on 72% of bird-nights, and continuously inactive periods of 10 days or longer were common. The extent of inactivity is similar behaviorally to that of hibernating small mammals. Roost selection, however, facilitated routine passive solar warming, and inactive birds exhibited a regular pattern of arousal on sunny days, followed by reentry into torpor at sunset. We argue that daily arousals are likely an adaptation to the circumstances that characterize surface dormancy. We hypothesize that the relationship between Ta and availability of flying insects at night, in combination with unique ecological aspects of arid regions, contributed to the evolution of multiday torpor use by poorwills.” (Woods et al. 2004:231)