When the sun sets, the active world of birds transitions from foraging and singing to resting and surviving. Most avian species are diurnal, active during the day, dedicating their nighttime hours primarily to rest. However, a specialized minority uses the cover of darkness for hunting, traveling, and feeding, making the night a time of great behavioral diversity.
Diurnal Birds: The Roosting Ritual
As twilight approaches, diurnal birds seek out a safe location for roosting, the avian equivalent of settling down for the night. These roosting sites are chosen for maximum shelter and security, often including dense evergreen shrubs, thick tree canopies, or sheltered cavities like hollow logs and building eaves. The precise location helps mitigate wind, rain, and the visibility of nocturnal predators.
Many small songbirds practice communal roosting, gathering in close proximity to share body heat and increase collective vigilance against threats. Roosting in a group offers a dilution effect, meaning the individual risk of being preyed upon decreases as the number of individuals increases. Larger roosts can also function as information centers, where birds may learn about the best foraging locations for the following day.
Birds physically manage sleep through the perching reflex, which allows them to grip a branch without conscious effort. When a bird lands and bends its legs, the flexor tendons in its feet automatically tighten and lock the toes around the perch. This reflex ensures a secure hold throughout the night, preventing the bird from falling even in deep sleep.
Their sleep itself can be a combination of bihemispheric sleep, where both sides of the brain rest, and unihemispheric slow-wave sleep (USWS). During USWS, one half of the brain remains awake and alert while the other half rests, allowing the bird to maintain awareness of its surroundings. The eye opposite the resting brain hemisphere remains open, maximizing predator detection, particularly when a bird is positioned on the edge of a group or in an exposed location.
Nighttime Survival Mechanisms
Surviving the long, cold hours of the night requires specific physiological and behavioral adaptations to conserve energy and maintain body temperature. One response is piloerection, or fluffing the feathers, which dramatically increases the layer of insulating air trapped close to the skin. This action creates an effect similar to wearing a thick down jacket.
To generate additional internal warmth, birds can shiver, rapidly contracting their large breast and leg muscles to produce heat. Birds also employ circulatory tricks, such as countercurrent heat exchange in their legs and feet. This mechanism transfers warm arterial blood to the cold venous blood returning from the extremities, keeping the core warm while minimizing heat loss from exposed feet.
A remarkable strategy, particularly for smaller species like hummingbirds and chickadees, is entering a state of torpor. Torpor is a controlled, temporary reduction in metabolic rate and body temperature, sometimes dropping the core temperature by over 20 degrees Fahrenheit. This state conserves energy when food is scarce, but because it makes the bird less responsive, it is typically only used for a few hours at night when predation risk is low.
The Truly Nocturnal Avians
While most birds sleep, a specialized group becomes active, taking advantage of the nighttime environment to hunt and travel. Owls are the most recognized nocturnal hunters, possessing numerous adaptations for thriving in the dark. Their large eyes contain a high density of light-sensing rods, granting them exceptional vision in low light.
Owls are masters of silent flight, thanks to specialized serrations on the leading edge of their flight feathers and a velvety surface on the trailing edge. This allows them to approach prey undetected. Their asymmetrical ear openings and facial discs funnel sound to pinpoint the location of small mammals, even beneath dense vegetation.
Nightjars, including nighthawks and whip-poor-wills, are insectivores with wide mouths used to scoop flying insects from the air.
Nocturnal Migration
The night is also a time for extensive travel, as many small diurnal songbirds, such as warblers and thrushes, undertake long-distance migrations under the cover of darkness. Migrating at night allows them to avoid diurnal predators and take advantage of cooler, calmer air, which reduces the energy expenditure required for flight. These migrants often navigate by the stars and the Earth’s magnetic field, resting and refueling during the day before continuing their journey.