Where Birds Sleep
All animals need to rest after activity. Birds are no exception. Diurnal birds, those that are active during daylight, spend about half of their lives resting and sleeping in a wide variety of styles. Some sleep on the ground, others in vegetation of some sort (trees, shrubs, marshes). Some sleep as they float on water; some rest in holes they have drilled in trees (or cavities other species created but later abandoned); some use underground burrows; some build elaborate covered nests just for sleeping. There are even some (pelagic) birds that sleep on the wing, high in the air above bodies of water. Some birds sleep alone, other in pairs, still others in large communities with hundreds, even thousands, of residents. Finding a secure sleeping space is just as important as finding the best place to build a nest for raising young. In cold climates, or high atop a tropical mountain where nighttime temperatures can drop to below freezing, birds must chose a sleeping or resting place where they can maintain their internal body temperature and not deplete their precious energy reserves. Otherwise, they would freeze and die, as would we.
Hummingbirds are warm-blooded (homeothermal) small animals, most weighing less than one eighth of an ounce. Our average sized hummers in Southern Arizona typically weigh about 3 grams. The way in which they survive the cold seems at first paradoxical. They abandon the effort to retain energy in order to conserve it. Since they are small, they have a large body surface area in proportion to their body size. This allows heat to dissipate easily. They temporarily become cold-blooded (poikilothermal) animals! Their metabolism is so high that they consume their body food equal to their body weight every day. On cold nights, they drop their body temperature to that equal to the ambient air temperature, shutting down many vital functions in the process. Like reptiles and amphibians, they expend less energy when cold. It is a form of brief hibernation, which has come to be called noctivation or torpidity.
Role of Temperature
Whether or not a hummer noctivates depends on several factors – the air temperature, its general health, its diet, whether it is breeding or not and possibly even its emotional state. In a large Brazilian aviary, some hummers noctivate every night, others rarely twice a week. Sometimes one of a pair of hummingbirds roosting together becomes torpid, but the other does not. The ambient air temperature is the chief controlling factor. The normal temperature of hummingbirds ranges from 102 to 108 degrees F when active or resting. In normal sleep, a hummer’s temperature drops from 4 to 9 degrees. During torpidity, it drops to that of the air around it. When the night temperature drops more than 12 degrees lower than the hummer’s normal daytime temperature, i.e. below about 93 degrees F, the bird may noctivate. Since birds lose moisture with every breath, and a resting hummer may breather 300 times a minute, torpidity helps our desert hummers conserve water as well as energy.
How much energy a torpid hummer saves depends on the external air temperature. When it 60 degrees F outside, it would save from one-fiftieth to one-sixtieth of the food it would burn at normal daytime temperatures. As the air continues to cool, most warm-blooded animals increase their activity and expend more energy to stay warm. The opposite is true for a noctivating hummingbird. The colder it gets, the more its metabolism slows, expending less and less energy. As the air temperature begins to warm in the morning, the bird increases it metabolic rate. When the air reaches about 95 degrees F, the hummer uses about the same amount of fuel, whether it is torpid or not. This is probably why no noctivation occurs above this temperature. When a female hummer is incubating eggs or has young nestlings, she does not noctivate. To do so would seriously retard the growth of the developing embryos and/or young.
If a sleeping bird’s body fluids congeal, it never wakes up. Some tropical hummers sleep and nest without shelter. In the high Andes, where nighttime temperatures are often below freezing, hummers seek warmer shelter inside a cove, mineshaft, crevice or building, often with a variety of other birds. If temperatures drop dangerously low, the hummers use metabolic processes (thermoregulation) to keep their body temperature from continuing to fall. Habitat has some influence. Birds that live high in the Andes, like the Andean Hillstar hummingbird, start to consume more oxygen at a lower temperature than hummers in warmer climates do.
Duration of Torpidity
The duration of torpidity varies from one bird to another. Some Brazilian hummers stay torpid for eight to fourteen hours, not awaking until the sun has had time to warm the air. Other species, like our Anna’s, are active before dawn or if in captivity, before the lights are turned on. The Common Poorwill, a member of the nightjar family, can be torpid for at least 88 days. Swifts are another group of birds that can use torpidity to their advantage while sleeping.
A potentially dangerous consequence of torpidity is that a noctivating hummer cannot fly. Its heart rate drops dramatically, and it appears lifeless. It may stop breathing for minutes at a time. If taken from its perch, it chirps weakly, moves its limbs slowly, and cannot regain its hold on the perch. Some could be held in the hand for more than an hour before they could move. Anna’s and Allen’s hummingbirds can fly fairly well after ten or fifteen minutes. Their body temperature rises at a rapid rate of one or two degrees per minute. A hummer’s temperature has to be at least 86 degrees before it can fly. This is a dangerous situation for hummers to find themselves in as it leaves them vulnerable to predators when torpid. But, it does enable them to survive life-threatening extreme cold weather nights.
Nature’s Survival Strategy
Other than hummers and poorwills who depend upon torpidity as a survival tool, some birds can be induced to enter a torpid state. This is true for owls, nighthawks, swifts, swallows, and titmice. Smooth-billed Ani, Turkey Vulture and Greater Roadrunner reduce their core body temperature on especially cold nights, but do not enter in deep torpor. Inca doves also drop their temperature in response to restricted food or water. Perhaps the length of time it takes to come out of torpidity is the reason the vast majority of larger birds do not use it. There would be no chance to escape the predator that shakes the perch, or heed the warning call of another bird, if torpid. Only smaller birds, those that have the most trouble maintaining their body temperatures through a chilly evening, noctivate in order to sleep soundly throughout the night.
Use of Feet During Sleep
Another interesting fact that comes into play when hummingbirds and passerines (perching songbirds) are asleep is the unique use of their feet. These birds, unlike birds that sleep on the ground or water birds, find a branch or perch to roost on for the night. Once they have selected their location for a good night’s sleep, the muscles in their feet grip the perch and lock onto that perch. This mechanism enables them to sleep soundly without having to use other muscles to stay upright or keep their balance. It also prevents them from falling or being blown off the branch or perch during harsh winds.
Dangers of Torpidity
Locking their feet muscles to their perch for sleeping purposes may also make sleeping birds easier prey for owls and other critters, like White-nosed Coati (Nasua narica), who are nocturnal hunters. Coati’s (related to raccoons and weasels) are known to travel in large groups in the evenings, roaming through the southern Arizona forests in search of their favorite edibles. This includes sleeping adult birds, nestlings or fledglings still in the nest, and even eggs in nests that haven’t yet hatched. Traveling in groups that sometimes number in the dozens, they noisily forage both through the trees, from branch to branch, and on the ground underneath the trees.
Give a Helping Hand
So, while we humans sleep soundly, and safely, in our homes – many bird species are susceptible to predators both during the daylight hours and during the evening hours while they are asleep. Providing housing for cavity using birds that frequent our yards can help birds survive that would otherwise be at greater risk. There are approximately 90 species of birds that are known for occupying bird houses or roosting boxes to sleep in, but hummingbirds are not among them.