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Hummingbirds and Torpor

Zoe Ann Hinds

 

Hummingbirds are amazing and extremely fascinating birds.  Prior to doing research and writing articles on them, I never imagined the sheer volume and variety of information which is available on these birds.  Sometimes, the idea for an article may come from a question someone has asked me about hummingbirds or from a particularly interesting discovery made during the course of doing further research on hummingbirds.  The idea for this article developed from a question a friend recently asked me about hummingbirds.  My friend wanted to know if hummingbirds sleep.  While I thought I knew the answer to this question, I told my friend that I wanted to do some further research on the subject before giving an answer to the question.  The article that follows is written as a direct result of research done on the subject.  I hope that you will find this article fascinating, useful, and also quite informative.

 

Yes, hummingbirds do sleep. They do so at night by entering into a state known as torpor, which is a state similar to hibernation.  Hummingbirds must enter this state to ensure that the birds won't actually starve to death before down. Torpor is a type of deep sleep where an animal lowers its heart and metabolic rate.  In a state of torpor the hummingbird lowers its metabolic rate by as much as 95%. A torpid hummingbird consumes up to 50 times less energy than when awake. The lowered metabolic rate also causes a cooled body temperature. A hummingbird's night time body temperature is maintained at a level which is barely sufficient to maintain life. This level is known as their set point and it is far below the normal daytime body temperature of 104°F or 40°C known for other birds of similar size. 

 

There are many reasons why the hummingbird must enter into a state of torpor.  Hummingbirds are among the smallest of all warm-blooded animals, and they also lack the insulating downy feathers that are typical for many other bird species. Due to their 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 in order for them to simply survive the night when they cannot forage. By entering into a state of torpor, a hummingbird is able to save enough energy to survive cold nights by lowering their internal thermostat. 

 

There are several types of torpor.  The various types of torpor are classified mostly by duration and season. If the state of torpor takes place over a long period 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. Tropical hummingbird species also have rigid metabolic demands and even they rely on daily torpor to conserve energy.

 

The sleep of torpid hummingbirds is a sleep that is as deep as death. According to the book, American Ornithology, written by Alexander Wilson in 1832, torpor is described as follows: "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."

 

It takes approximately 20 minutes for a hummingbird to awaken from a state of torpor.  When awakening, the hummingbird's heart and breathing rates increase and they vibrate their wing muscles. The bird's blood supply will be warmed by heat being generated by their vibrating muscles or from shivering.  Shivering helps to warm the hummingbird's body temperature by several degrees each minute and the bird awakens with enough energy reserves to see him through to his first feeding of the morning.  Hummingbirds reliably awaken from torpor one or two hours before dawn without any known cues from the environment.  It appears that the bird's internal circadian clock is responsible for signaling when it is time to awaken from the state of torpor. 

 

Hummingbirds are not the only birds known to enter into a state of torpor.  Other birds that are known to enter into a state of torpor include swallows and swifts. Scientists think that most small birds living in cold regions, such as chickadees, rely on torpor to survive long cold nights. Rodents, bats and other small mammals typically show some form of regulated hypothermia during cold weather, and these animals can only rely upon daily torpor during the winter months when they are not breeding. For hummingbirds, noctivation is possible on any night of the year. It is the hummingbird's ability to enter into a state of torpor that will literally prevent it from starving to death before dawn.

 

 

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