Hummingbirds and Torpor

A flash of scarlet and emerald zooms past me as I poke my sleepy head out of the kitchen door, a vibrant splash of summer color against the sullen winter sky. Suddenly, an indignant Anna's Hummingbird, Calypte anna, confronts me, beak-to-nose, demanding his breakfast. Shivering, I retreat quickly into the kitchen to prepare warm sugar water for my feathery guest.

Hummingbirds are classified into the avian family, Trochilidae, which is from the Greek word, trochilos, or "small bird." In fact, the smallest avian species alive today is the thumb-sized Bee Hummingbird, Mellisuga helenae, found exclusively on the island of Cuba. With a total length of 2.25 inches (5 centimeters) and a weight of 0.07 ounces (2 grams), this tiny bird can comfortably perch on the eraser at the end of a pencil.

There are more than 330 described species of hummingbirds, and occasionally a new species is discovered by ornithologists and added to the list. Even though most people think of them exclusively as tropical birds, hummingbirds are found in diverse habitats, ranging from the wettest to the driest, from sea level to over 14,000 feet (4400 meters).

The greatest diversity of hummingbird species is the neotropics (New World tropics) but many species live in or migrate to temperate zones in the United States and Canada to breed. Sometimes, for reasons that are not entirely clear, individual birds remain behind for the winter, and sometimes, they survive. Thus, as average seasonal temperatures increase, hummingbirds are increasingly becoming established as year-round residents outside of their traditional ranges. Anna's Hummingbird is one species whose range has expanded steadily northward as seasonal temperatures have become milder. Thus, this bird is now a common year-round resident along the northwestern coast of the United States and even into some parts of Canada.

As most people know, hummingbirds feed on flower nectar, which is a tempting "gift" of high-energy sugars provided by flowers in exchange for pollination. In addition to nectar, hummingbirds also consume large quantities of small insects, which are full of higher-energy fats as well as essential proteins. Because of their tremendous metabolic requirements, hummingbirds have voracious appetites. Equivalent to the average human consuming an entire refrigerator full of food, hummingbirds eat roughly twice to thrice their own body weight in flower nectar and tiny insects each day.

Besides being among the smallest of all warm-blooded animals, hummingbirds also lack the insulating downy feathers that are typical for many other bird species. Due to their combined characteristics of small body size and lack of insulation, hummingbirds rapidly lose body heat to their surroundings. Even sleeping hummingbirds have huge metabolic demands that must be met simply to survive the night when they cannot forage. To meet this energetic challenge, hummingbirds save enough energy to survive cold nights by lowering their internal thermostat at night, becoming hypothermic. This reduced physiological state is an evolutionary adaptation that is referred to as torpor.

Torpor is a type of deep sleep where an animal lowers its metabolic rate by as much as 95%. By doing so, a torpid hummingbird consumes up to 50 times less energy when torpid than when awake. This lowered metabolic rate also causes a cooled body temperature. A hummingbird's night time body temperature is maintained at a hypothermic threshold that is barely sufficient to maintain life. This threshold is known as their set point and it is far below the normal daytime body temperature of 104°F or 40°C recorded for other similarly-sized birds.

Research shows that this set point is actively maintained by the bird's internal thermostat. "If you try to cool an animal down below this new set point, it will generate enough body heat to maintain that set point," says Sara Hiebert, hummingbird expert and associate professor of biology at Swarthmore College in Swarthmore, Pennsylvania.

There are several types of torpor, classified mostly by duration and season. For example, when torpor takes place for long periods of time during the winter, it is known as hibernation. However, unlike hibernation, hummingbird torpor can occur on any night of the year so it is referred to as daily torpor or noctivation. Because tropical hummingbird species also have rigid metabolic budgets, even they rely on daily torpor to conserve energy.

Torpid hummingbirds exhibit a slumber that is as deep as death. In 1832, Alexander Wilson first described hummingbird torpor in his book, American Ornithology; "No motion of the lungs could be perceived ... the eyes were shut, and, when touched by the finger, [the bird] gave no signs of life or motion."

Awakening from torpor takes a hummingbird approximately 20 minutes. During arousal, heart and breathing rates increase and hummingbirds vibrate their wing muscles. Heat generated by vibrating muscles, or shivering, warms the blood supply. Shivering is sufficient to warm the hummingbird's body by several degrees each minute and the bird awakens with enough energy reserves to see him through to his first feeding bouts of the morning. Interestingly, hummingbirds reliably awaken from torpor one or two hours before dawn without any discernible cues from the environment. Thus, it appears that the bird's internal circadian clock triggers arousal.

What are hummingbirds doing during those pre-dawn hours when they are warm but not yet active? "One suggestion is that they might be using this time to sleep," explains Hiebert. "Although there is some evidence that torpor is an extension of slow-wave sleep, there is also evidence that the body is too cold during torpor for the normal functions of sleep to occur."

Torpor is not limited to hummingbirds; it has also been observed in swallows, swifts and poorwills. Additionally, scientists think that most small birds living in cold regions, such as chickadees, rely on torpor to survive long cold nights. Interestingly, even though rodents, bats and other small mammals typically show some form of regulated hypothermia during cold weather, these animals can only rely upon daily torpor during the winter months when they are not breeding. In contrast, noctivation is possible on any night of the year for hummingbirds. Because daily energy balance is progressively more difficult to maintain as body size decreases, hummingbird torpor is a finely tuned evolutionary strategy that preserves these birds' daily metabolic budgets.

"Hummingbirds are the 'champions' of this kind of energy regulation because they have to be," concludes Hiebert.

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Many thanks to Sara Hiebert for allowing me to interview her for this story.



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tags: torpor, ornithology, birds, hummingbirds, avian