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Below is a explanation of what torpor is and how Anna's hummingbirds use it to survive cold weather, and how they need it, as they have expanded their territory further northward during just the last hundred years or so. Torpor is just a shorter form of hibernation. Also explained are three factors that have allowed and encouraged them to live further north and spend the winter in those newer northern locations.
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.
Of the four Pacific Northwest hummingbirds, Anna’s is the only one that doesn’t always migrate south to warmer climes in the winter. Rufous, Calliope, and Black-chinned Hummingbirds have enough sense to leave for Mexico and the Gulf Coast by late summer. Most of the males are gone by early July.
Anna's hummingbirds have been overwintering at higher latitudes only for the last few decades. Prior to the 1930s, it nested no farther north than San Francisco Bay and was not reported north of the Oregon border until 1944. The bird reached Seattle in 1964 and today breeds on Vancouver Island and is found in southeastern Alaska regularly.
Their journey north appears to have begun with the appearance, and then northward establishment, of another species: the blue gum eucalyptus tree from Australia. First introduced to southern California in the 1870s for shade, lumber, and railroad ties, and later used for lumber and orange-grove windbreaks, the tree is now naturalized in the coastal areas of southern California and the San Francisco Bay region. Areas of the state that were once treeless plains are now savannahs or long-abandoned plantations of blue gum.
The tree’s nectar-rich flowers bloom in the winter. Anna’s Hummingbird is one of only two native wildlife species that appear to find value in the tree. The Monarch butterfly, which uses it as a winter roost, is the other. Taking advantage of a developing urban horticulture in the Los Angeles Basin, Anna’s found it could now live year-round in the lowlands of southern California and later move north to the Bay area as blue gum groves there matured.
Blue gum trees aren’t common in Oregon and Washington, so why did Anna’s Hummingbird continue north? Part of the answer is hanging off my back porch. Nectar feeders in urban areas provide a super-rich food source. Why fly around licking dewy drops of nectar from scattered flowers full of bees when a whole quart of the stuff is just hanging there? It may not be entirely natural, but it is the same sucrose sugar that the hummingbird favors.
Add to that the growth of some urban areas of the Pacific Northwest. Development has replaced the native conifer forest with incredibly rich and diverse garden flowers, many of which bloom earlier or later than native flowers, providing a longer plant-nectar feeding period for hummingbirds. Some flowers, such as winter jasmine, viburnum, sweet box, witch hazel, Oregon grape, and heather, even bloom in winter.
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