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 TheAudubon 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.”
Torpidity in other birds
Hummingbirds are not the only birds that can become torpid. The Common Poorwill is a master, maintaining a state of torpidity for 88 days. Swifts (which are related to hummingbirds) can enter torpidity, and it can be induced in owls, nighthawks, swallows, and titmice. Although they do not enter deep torpor, Smooth-billed anis, Greater roadrunners, Inca doves and Turkey vultures all can decrease their body temperature on cool nights.
As an emergency measure
In 1950 Oliver L. Pearson noted that Anna’s and Allen’s hummingbirds enter torpidity. He pointed out that a hummer that becomes torpid overnight saves a lot of energy. In 1963 Lasiewski observed 67 individuals of five different hummer species – Costa’s, Anna’s, Allen’s, Rufous, and Calliope – and concluded that hummingbirds do not become torpid every night. It is probably an emergency survival method used when energy reserves are low. Presumably, hummers cannot survive much below freezing at night. They must locate to a warmer location or find shelter, like in a cave or some other enclosed area, as Pearson reported in 1953.
However, there are reports of hummers flying about and surviving in below freezing weather. A Ruby-throated hummer was seen in the last two weeks of May 1936 in Canada when the temperature was only 22-28 degrees. From December 3, 1963 to January 3, 1964, a female Ruby-throat visited a North Carolina garden. The nighttime temperature dropped below freezing during 28 of those nights and below 20 degrees on seven consecutive evenings. The little bird almost totally depended on a nectar feeder that was put out for it. No one found its protected nighttime roost.
Feeders are helpful
In the same vein, the internet offers records of a Rufous hummingbird spending her winters in central Texas. Entries like the one following point to the use of feeders and their importance in maintaining the bird through ice storms and hard freezes. “There was a 24 degree freeze on November 16 (1997), but our Rufous was at the frozen bottle early on. I used the warmth of my hands to thaw the solution to where it could be used, but also put out a second bottle in a sunny location – immediately preferred.” If you want more information, check out the Purola Bird Reports site at http://uts.cc.utexas.edu/~harms/birds.html
To maintain the temperature of her eggs or young, an incubating or nesting female hummingbird does not lower body temperature and become torpid. The nestlings do not enter into torpidity either. The insulated nest retains enough warmth to keep them from the cold.
A different strategy
The Magnificent hummingbird uses a different strategy. This hummer stores insects and nectar in a well-developed crop. In view of Lasiewski’s discovery that hummers do not always enter into torpidity, Weymouth et al (1964) found that storing food in the crop helps maintain the Magnificent through a cold winter’s night, much as finches often fill their crops with seed before roosting in the far north.
Truly unique birds!
Hummingbirds never fail to interest and amaze those who focus their attention on them. They truly are among the most unique of birds, anywhere. As such, they are one of a few birds that can raise and lower their body temperatures and metabolic rates to survive in the extremes of winter in North America. Truly incredible!