Most birders have seen the occasional duck taking a nap on the shoreline or an owl roosting during the day, but it is generally unusual to see sleeping birds. They do need their rest, however, but a bird's sleep is very different from a human's. Learning more about how birds sleep shows just how amazing these creatures are.
Where Birds Sleep
Birds are rarely seen sleeping because they don't typically nap in the open. Instead, they choose safe, concealed locations where they are protected from predators and the elements. Those locations are generally off the ground to avoid creeping predators, and even terrestrial birds, such as wild turkeys, often roost out of reach in trees. Smaller birds nestle into dense brush or foliage that provides adequate shelter. Many birds seek out cavities such as unoccupied birdhouses or roosting boxes, a hollow snag, a shallow cave or cliff crevice, a chimney, or just the deep crook of a tree where they are more protected so they can sleep safely.
Waterfowl and wading birds often sleep on the water, floating safely out of reach of predators or opting for small islands as roosting spots. Should a predator approach, the splashing noises and vibrations of moving water will easily alert the birds.
One place birds do not generally sleep is in the nest. While a bird that is actively incubating eggs or keeping small chicks warm may nap on the nest, once the birds are grown they do not return to the nesting site to sleep. After the nesting season, a nest is often coated with feces, bits of leftover food, shed feathers, and other debris. The nest may also be infested with mites and it is often falling apart from the vigorous use of multiple hatchlings. This makes it unsuitable for sleeping, and while some birds will return to birdhouses for winter roosting, they generally only do so if the birdhouse has been appropriately cleaned and winterized to be as useful as possible.
How Birds Sleep
Unlike humans and other mammals that enter a state of relatively complete unconsciousness while sleeping, birds can more carefully control their sleep. Birds often use unihemispheric slow-wave sleep (USWS), literally sleeping with one eye open and only half their brain resting at once. The other half of the brain is alert, able to note danger if needed. While the exact way birds control their sleep patterns has not been well studied, it has been shown that the more protected a bird feels when sleeping, the more likely it is to sleep deeper. If the situation is more uncertain, however, the bird will sleep more lightly and is more likely to use USWS. It is believed that some migrating birds or aerial species such as swifts or albatrosses may even use USWS in flight, literally sleeping in the air.
Sleeping in flocks is another defensive strategy many birds use. By roosting communally, with some species creating nighttime roosts of thousands of individuals, there are more birds to notice predators as well as more targets should a predator attack, giving each individual bird a greater chance of survival. In winter, many birds, particularly small passerines such as chickadees, ****, and bluebirds, roost together in confined spaces to share body heat and survive lower nighttime temperatures.
When birds sleep, they protect vulnerable body parts by burying them in their feathers. A bird's feathers create insulating air pockets that help it keep warm, and by tucking feet or the bill into the feathers, less body heat is lost. When a bird's bill is buried deeply in its feathers, it is also able to breathe air warmed by its own body heat.
Another adaptation birds have for safe sleep is the construction of their feet and legs. A flexor tendon contracts the bird's toes and talons when the legs are bent, such as when a bird is nestled down for roosting. This means the automatic, at rest position of the foot is for the talons to be tightly locked around a perch, making it impossible for the bird to fall while sleeping. The tendon only releases when the bird voluntarily straightens its legs, as it would for taking off.
There is little data available about how long birds sleep at once, but studies indicate that birds may sleep longer during longer nights. This may be because diurnal birds are not able to forage or engage in other activities without sufficient light, and so sleeping is the next natural item on their to-do lists. Birds are able to "power nap" during the day, however, and can catch up on sleep on longer days whenever they are in a safe, secure spot.
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