The mercury in Salt Lake City is beginning to poke its silver snout into the positive Celsius digits, breaking a cold snap that's kept me cowering indoors for most of the past three weeks. Outside, the deep torpor of reptiles and amphibians will last well into March. Visible mammal life is restricted to a couple of hardy Red Squirrels (Tamiasciurus hudsonicus) that venture briefly from their dreys during the warmest hours to fuel up on cached pine nuts, and the more numerous House Mice (Mus musculus) that find relative warmth in my woodshed and composter. Meanwhile, birds, cloaked in insulating feathers, are conspicuous through the coldest days. Best protected are the gulls, loons, grebes, and others whose foraging requires prolonged contact with cold water. The insulating and water-repelling properties of duck and goose feathers are legendary, and they leave their owners well-equipped to float, warm and dry, upon the iciest waters, propelled by naked feet that can remain submerged without much heat loss. The arteries and veins of the legs run alongside one another, allowing heat from the arterial blood to diffuse into the cold venous blood, preventing major heat loss through the feet. Such heat-exchange systems are fairly common in nature, and are most sophisticated in certain poikilothermic animals. A device based on the same principle ventilates my studio with outdoor air that's cooled in the summer and heated in the winter from the exhaust duct. The circulation to the feet is also temperature-regulated; in extreme cold, blood flow is restricted.
Being close to the brain, the naked parts of a bird's head are especially prone to dangerous heat loss. Temperature is regulated by tiny connections between arteries and veins, called arteriovenous anastomoses, in the eyelids and bill, and by a heat-exchange system beneath each eye called the Rete Ophthalmicum, which is peculiar to birds. Blood to the bill is also regulated by temperature-sensitive vein restriction. These specialized circulatory structures are especially well-developed in Arctic diving-ducks like Scoters (Melanitta spp.) and Eiders (Somateria spp.). A recent news story told of a Ring-necked Duck (Aythya collaris), shot by a hunter and refrigerated, that regained consciousness. The surprising part of the story was not that the bird survived the modest cold of a refrigerator for two days, or even the limited oxygen, but that the hunter evidently planned to eat her after leaving her ungutted for two days.
In cold weather, birds can generate a substantial amount of heat by shivering, particularly in their pectoral muscles. Simple behavior like facing the wind, fluffing the feathers, pulling one leg up into the flag feathers, or tucking the bill into the feathers of the back, can do much to minimize heat loss. Group huddling during cold weather is a common tactic for keeping warm; even among birds that aren't particularly social. Warm microhabitats may also be sought, including caverns, thick conifers, tree holes and buildings. Many pheasants, grouse and partridges are particularly fond of tunneling into deep snow.
Because of their higher metabolic rates, birds are forced to spend more time and energy foraging than the average mammal—an expensive activity when over 90% of your metabolic energy is going toward maintaining body temperature. The ability of birds to adjust their metabolism is just beginning to be understood. When Edmund Jaeger discovered a hibernating Common Poor-will (Phalaenoptilus nuttallii) in 1946, ornithologists were forced to re-think their notions of bird metabolism. By lowering their body temperature by as much as 35ºC, Poor-wills are able to devote far less energy to keeping warm. These remarkable birds can actually fly while their body is as cold as 27.4ºC. (normal temperature is about 40ºC.). As researchers have studied this phenomenon, they've found that Poor-wills are particularly prone to torpor, and regularly enter it for short periods during cool summer nights. It appears that many goatsucker species, including the Whip-poor-will (Caprimulgus vociferus), are incapable of metabolic adjustment, while others, like the European Nightjar (Caprimulgus europaeus) and even the one-pound Tawny Frogmouth (Podargus strigoides), seem to enter torpor regularly, although so far, no bird has been found to “hibernate” as effectively as the Poor-will.
Unlike their relatives the nightjars, owls appear to be unable to regulate their metabolism in any meaningful way. My own observations lead me to consider Barn Owls (Tyto alba), the most nightjar-like of American owls, to be one of the most cold-sensitive bird species that regularly winter in my area. During severe cold, it's not uncommon to find their frozen corpses hanging in trees. How some other owls are able to tolerate cold as well as they do is still a mystery to me. Great Horned Owls (Bubo virginianus), our earliest nesters, are already courting. As a kid, I remember waiting for the school bus, listening to their courtship hoots in -20ºF weather. The ability of young altricial birds to thermoregulate is generally next to zero, but the mortality of Bubo chicks to cold seems to be inconsequential.
Besides the nightjars, many swifts and hummingbirds are able to drop their core body temperatures radically. Because of their high metabolic rate (a resting hummingbird burns about 5 times as many calories as a resting sparrow), hummingbirds would have trouble surviving their nightly fast without becoming torpid. At night, a hummer can drop its body temperature by 20ºC or more, breathing but a couple of times each minute, instead of the regular 400, its 500 heartbeats reduced to 30. An hour before dawn, the bird begins to stir, shivering to raise its core temperature. Within half an hour, its metabolism is back to normal.
This nightly torpor enables hummers not only to survive a cool summer night, but rather harsh extremes. The particularly foolish young Black-chinned Hummingbird (Archilochus alexandri) pictured above failed to migrate (possibly because it found backyard hummingbird feeders still being filled), and was photographed before an attractive little snowbank backdrop by my friend Hillary on January 7. Hillary put out warm sugar water for the bird, but the mercury plummeted shortly after the photo was taken, and I'm sure the poor little fellow's torpor soon became permanent.
Only a fraction of bird species have been tested for metabolic regulation, but some degree of ability seems to be fairly widespread. Among passerine birds, swallows and sunbirds (Old World convergent analogs of the hummingbirds) appear to achieve the lowest temperatures. Some degree of metabolic reduction has been recorded in chickadees, doves, condors, birds of paradise, and numerous other groups. Some diving birds appear to enter similar states while submerged. Hawks and falcons will lower their basal temperature during cold weather if they become too lean, although this is a stressful last-ditch effort at avoiding starvation, that falconers have understood for centuries.
upper: RED-BREASTED MERGANSERS (1989) acrylic 7" x 9"
second: Poor-will photographed by CPBvK in Tooele Cp., UT 1983
third: HUNGRY EYES--GREAT HORNED OWLETS (2005) acrylic 30" x 22"
fourth: BLACK-CRESTED COQUETTE (Lophornis helenae) (1999) acrylic 10" x 8"
lower: Black-chinned Hummingbird photographed by Hillary White in Salt Lake Co., UT 1-'07