Every birder is used to seeing birds as frenetic bundles of energy. They're constantly flying, feeding, bathing, singing, nest-building, preening, and doing other activities. No matter what they are doing, birds are almost always in motion, with only brief pauses to perch and look about before making their next move. But even the most active bird needs to rest occasionally—so how do birds sleep?
When Birds Sleep
Most birds sleep at night. Species that are active during the day with all their regular activities are called diurnal birds, and they rest more after dark. Nocturnal birds, on the other hand, are most active at night, and they sleep during the day. Owls are the most familiar nocturnal birds, but night-herons, kiwis, poorwills, frogmouths, nightjars, pauraques, and woodcocks are also largely nocturnal.
Even diurnal species don’t just sleep at night, however. Many birds will catch brief naps during the day, often when the temperatures are hottest and it is better to stay still, or after activities when they need to rest. After a large meal, for example, many raptors will rest and nap as they digest. Nesting birds may also take brief naps while they incubate their eggs, or birds might nap after a vigorous bath while their plumage dries. Birds might also nap during bad weather, such as an afternoon thunderstorm, when it isn’t safe or possible to be out and about with other activities.
Where Birds Sleep
Different birds sleep in different places, generally in sheltered locations where they are more difficult to find. Small birds will nestle deep into brush or in thick tree foliage where they are protected from both predators and the elements, even if it is just a sturdy crook in a tree branch. Some birds, particularly quail or other terrestrial species, will snuggle into slight hollows in the ground under thick clumps of grass or brush. Wading birds and waterfowl will often sleep right on the water. They will either float or stand surrounded by water. Splashes and waves would awaken them if a predator approaches.
Cavity-nesting birds may sleep in cavities as well, tucking themselves into rock crevices, hollow trees, or empty bird houses or roost boxes for safe sleep. This is especially true in winter, when the extra shelter will help keep them safe from snow and ice and will conserve body heat. Several small birds, such as a flock of chickadees, bluebirds, or wrens, may snuggle together in winter to share the cozy space and body warmth.
Learn more: Where Do Birds Sleep at Night?
How Birds Sleep
Birds do not often go into the same deep unconsciousness as humans do when they sleep. Instead, birds have greater control over their sleep cycles, and typically use unihemispheric slow-wave sleep (USWS). In this type of sleep pattern, only half of the bird’s brain is at rest, while the other half is alert for predators and other threats. This allows birds to stay protected even as they sleep. Some migratory birds or those who remain aloft for days at a time, such as swifts and albatrosses, even use USWS in flight, and actually sleep while they are flying.
Birds that feel more protected as they sleep are able to sleep more deeply and for longer periods. In extreme cases, such as on very cold nights, some birds will even enter a state of semi-hibernation called torpor, where their body functions are significantly lowered. Not only is this a deeper type of sleep, but it also helps conserve energy so the bird can survive a longer period of inactivity. Hummingbirds regularly use torpor, and swifts, doves, nighthawks, and even roadrunners use torpor occasionally as well.
See also: What Is Torpor?
Just as humans may sleep in many positions, birds also have different sleep positions. Some birds will simply crouch down and lay on the ground or a branch, while others will sleep standing up. Some waterbirds, such as ducks, geese, and swans, will float while sleeping, while albatrosses and petrels may sleep while flying. There are even small parrots that dangle upside down to sleep!
When a bird sleeps, it often tucks bare, vulnerable bits into its feathers to conserve body heat. This is why many birds tuck their bills into their feathers for a nap, or a raptor might tuck one leg up into its feathers as it sleeps while perched. Flamingos are masters at sleeping while standing on just one leg, with their bills tucked into their feathers.
See also: A 5 Step Plan for Nighttime Birdwatching
One physical adaptation of birds helps keep them safe and secure as they sleep. Passerines have a strong flexor tendon in their feet, and that tendon’s relaxed, automatic position keeps the talons clenched. This ensures that a bird will not lose its grip as it sleeps, keeping it safely perched. Only when the bird deliberately straightens its legs to stand or take off will that tendon relax and the talons release.