Our coldest nighttime temperatures here in Southern Arizona occasionally drop to the freezing mark, or even below. Last winter, we had the coldest winter weather since records have been kept. Luckily, this winter won’t be as cold. Many of our customers have asked us how it is that tiny, fragile hummingbirds are able to survive such cold night temperatures.
Most North American hummingbirds are migratory. Do they simply fly south from their northerly breeding territories and ranges for the winter when the weather is cold and their food sources are nearly exhausted? In most of North America, the hummers leave long before the prolonged cold sets in. Many researchers believe that lack of food is not the main reason hummers (as well as other birds) migrate. It is believed that the number of daylight hours and raging hormones are more critical factors in determining when hummers migrate south.
The Tucson basin has a temperate enough winter climate that this region becomes the destination of their southern migration for some hummer species. Others continue farther south. In recent decades, due in part to the relatively mild climate in winter and the great increase of backyard nectar feeders and nesting materials, many species are seen here throughout the coldest of months and some species seem to have adapted well to year round living in Southern Arizona. Anna’s hummingbird is a common hummer that breeds here in winter.
In Southern Arizona, we have hummingbirds every month of the year, every day of every month. We can’t complain about any lack of hummers, especially since 21 species have been recorded here.
On particularly cold evenings, Anna’s (and other species) enter a state similar to hibernation called torpidity. John K. Terres, author of The Audubon Society Encyclopedia of Birds, defined torpidity as “a state of inactivity that is brought about by certain physiological changes – greatly lowered heart rate, breathing rate, and metabolism, and a greatly reduced response to external stimulation.” While hibernation is generally thought of as a long-term period (an entire winter – bears are a good example), torpidity is very short term, usually only overnight or for several nights in a row.
During cold snaps, small birds lose body heat more quickly than large birds. Likewise, small birds overheat more quickly than large birds when subjected to temperatures greater than their own body temperatures. Some cold climate small birds like Chickadees are equipped with down feathers. They fluff their feathers up and trap more insulating air when it is cold. They also shiver like we humans do to cause muscular contractions that generate heat. While hummingbirds have more feathers overall than larger birds, they don’t have down feathers. They can only maintain their body temperature by increasing heat production.
How torpidity works
The problem is that hummers have little energy reserves due to their extremely high metabolic rate. They cannot survive by shivering for very long. Instead, many enter a state of torpor. They drop their metabolic rate to 1/50th of what it would be at normal body temperature. The rate of water loss by evaporation decreases to one-third to one-tenth of the norm. The smaller the hummer, the more rapidly it enters and emerges from a torpid state. While torpid, the hummingbird’s heart rate varies with its body temperature, ranging from 50 to 180 beats per minute. Normal heartbeat of active hummers can be as high as 1200 beats per minute. In their state of torpidity, breathing becomes irregular, with long periods of no breath at the lowest temperatures.
Alexander Wilson, the father of American ornithology, first noted torpidity in 1810 when he observed a Ruby-throated hummingbird that took over half a day to come out of a torpid state. In 1861, British artist John Gould wrote of two species of South American hummingbirds that became torpid in his laboratory at 63-70 degrees. Tropical hummingbirds have very little cold tolerance. They sat on their perches with heads drawn into their shoulders, and “showed…no spark of life; they could be moved about and laid on a table like so many dried skins.”