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 burrowed out of the ground; some build elaborate covered nests just for sleeping. There are even a few birds that sleep high in the air. Some birds sleep alone, others in pairs, still others in huge communities with thousands of residents. Finding a secure sleeping place 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 below freezing, birds must chose a sleeping or resting place where they can maintain their internal body temperature and not deplete their energy reserves. Otherwise, they would freeze and die, as would we.
Hummingbirds are warm-blooded (homeothermal) small animals, most weighing less than one quarter of an ounce (7 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 equal 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.
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° F when active or resting. In normal sleep, a hummer’s temp drops from 4 to 8° F. During torpidity, it drops to that of the air around it. When the night temperature drops more than 12° F lower than the hummer’s normal daytime temperature, i.e. below about 93° F, the bird may noctivate. Since birds lose moisture with every breath, and a resting hummer may breathe 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’s 60° F outside, it would save from one-fiftieth to one-sixtieth of the food it would burn at normal daytime temperature. 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 temperature begins to warm in the morning, the bird increases its metabolic rate. When the air reaches about 95° 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 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 cave, mineshaft, crevice or building, often with a variety of other birds. If temperatures drop dangerously low, the hummers use metabolic processes (thermoregulate) 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.
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 had time to warm the air. Others, like 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.
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 your 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° F before it can fly. This leaves them vulnerable to predators when torpid.
Other than hummers and Poorwills, some birds can be induced to enter torpidity— owls, nighthawks, swifts, swallows and titmice. Smooth-billed Ani, Turkey vulture and Greater roadrunner reduce their body temperature on cold nights, but do not enter into 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 to heed the warning call of another bird, if torpid. Only smaller birds, who have the most trouble maintaining their body temperatures through a chilly night, noctivate.