Winter World_ The Ingenuity of Animal Survival - Bernd Heinrich [56]
The torpor option for conserving energy was little known until the 1950s, when Jon Steen (1958) in Norway examined the metabolism of titmice and five species of common finches captured one winter just outside his lab near Oslo. At night the birds reduced their heat loss by balling up—tucking their heads into their back feathers—but they also reduced heat production by lowered body temperature at night. However, when plenty of food was available, the birds maintained normal body temperature at night, shivering often the whole time, while sound asleep.
Torpor of one degree or another is a well-known strategy for conserving energy of many of our winter birds. No bird in North America has been as well studied in that regard, and is as familiar to many people, as is the black-capped chickadee (Parus atricapillus). This 10-to 12-gram bird takes sunflower seeds from bird feeders all winter, and no walk in the snow-filled winter woods is complete without at least one run-in with a family flock of these small, tame, and inquisitive birds as they search for food. Day in, day out they are active, no matter how cold the weather. In Alaska the birds show a peak of food-hoarding activity in November (Kessel 1976), similar to other tits (Nakamura and Wako 1988). Nevertheless, how they maintain an energy balance during long cold night fasts was a mystery until they were investigated in the early 1970s by Susan Budd Chaplin at Cornell University.
Chaplin’s study of chickadees in the Cayuga Lake Basin in New York focused on the most critical time for cold survival: winter nights. She began by determining energy expenditure of the birds as they regulated their normal day-active body temperature of 42°C at air temperatures as low as 0°C and as high as 30°C. She recorded the chickadees’ rates of energy expenditure (oxygen consumption) when they were placed into constant temperature in a sealed chamber in the lab. As in all other homeotherms (animals that regulate a high and constant body temperature), metabolic rates were predictably high at low air temperatures, to make up for increased rates of heat loss. Given the hours of darkness per night, Chaplin could calculate how many calories of energy reserves a bird might require to enable it to remain fully heated over the duration of a night, and then she compared to see how that number correlated with the calories stored in the body’s fat reserves.
Fat has more than twice the caloric content per unit weight as carbohydrates (such as sugar and glycogen). Fat is therefore the fuel of choice for long-distance travel and for other long durations of exercise, such as all-night nocturnal shivering of sleeping birds (Marsh and Dawson 1982). The chickadees’ fat reserves were determined by collecting birds in the woods (by shotgun) in the evening and again in the early morning, then chemically extracting their fat contents to see how much energy they used during the night. Body fat in the evening measured 7 percent and only 3 percent in the early morning. That is, the birds fattened up throughout the day and then burned off their fat to produce heat to keep warm during the night. Winter birds had higher metabolism (Rising and Hudson 1974) and generally maintained twice the fat content of fall and spring birds. Indeed, the ability in any birds to put on body fat in significant amounts is generally restricted to only those that need it for migration or surviving