The common poorwill, Phalaenoptilus nuttallii, is a small, nocturnal bird native to western North America. The smallest North American nightjar, it measures about 18 centimeters (7.1 inches) and weighs 36-58 grams. Its mottled gray, brown, and black plumage provides excellent camouflage in dry, open habitats like grasslands, shrublands, and stony desert slopes from British Columbia to northern Mexico. As a nocturnal insectivore, it hunts moths, beetles, and grasshoppers.
A Bird’s Unique Winter Survival Strategy
The common poorwill exhibits a remarkable adaptation for surviving periods of scarcity and cold, a behavior almost unparalleled among birds. It is the only bird species known to enter a prolonged state of torpor for weeks or even months.
This deep, inactive state is colloquially referred to as “hibernation” for the poorwill. The Hopi people historically called the bird “hölchko,” meaning “The Sleeping One.”
Torpor, a state of decreased physiological activity, allows the poorwill to conserve energy when environmental conditions are unfavorable.
While other bird species, such as hummingbirds, can enter brief, daily bouts of torpor, the common poorwill’s ability to sustain this state for extended durations sets it apart. This adaptation provides a survival advantage, particularly in its arid and often harsh habitats where food resources become scarce during colder months.
The Biology Behind the Poorwill’s Slumber
Entering torpor involves significant physiological changes for the common poorwill. Its body temperature can drop dramatically, sometimes by 4 to 35 degrees Celsius, reaching as low as 5°C (41°F). This profound temperature reduction is accompanied by a drastic decrease in metabolic rate, with oxygen consumption falling by over 90 percent.
The bird’s heart rate slows considerably, potentially dropping from a typical 130 beats per minute to as few as 10 beats per minute, and its respiration becomes almost undetectable.
This deep slumber is primarily triggered by environmental cues such as sustained cold weather and the scarcity of its insect prey. Poorwills do not enter torpor during their breeding season but can utilize it at other times of the year when conditions demand it.
When seeking a place for torpor, they often conceal themselves in rock piles, crevices, or sheltered depressions, sometimes under dense vegetation or in hollow logs. They frequently select locations with a southern or southwestern exposure, which aids in passive rewarming from sunlight.
Why This Adaptation is Crucial for Survival
The common poorwill’s unique adaptation of prolonged torpor is essential for its survival in challenging environments. As an insectivorous bird, its primary food source, nocturnal insects, becomes scarce or unavailable during colder periods.
Without the ability to significantly reduce its energy expenditure, the poorwill would face starvation and succumb to the harsh conditions.
By entering this state, the bird minimizes its energy consumption, allowing it to endure periods of food shortage and low temperatures that most other bird species cannot withstand.
This strategy enables the common poorwill to persist in its specific habitats, which are often characterized by extreme temperature fluctuations and unpredictable resource availability. The ability to “sleep through” the lean times is a highly effective mechanism that ensures the species’ continued presence in these arid regions.