Winter World_ The Ingenuity of Animal Survival - Bernd Heinrich [57]
Given Susan Chaplin’s figures, chickadees are already close to an energy edge at 0°C, far from the lowest temperatures they might encounter during any winter night. Unlike some seed-eating birds, who usually have more food calories available and who put on a lot more fat overnight (Reinertsen and Haftorn 1986), Chaplin’s chickadees did not have sufficient caloric reserves in fat to make it through a night at 0°C, if they continued to regulate the same body temperature at night as during the day. However, she discovered that, unlike the seed-eaters, they stretched their fat reserves by lowering their body temperature to 30° from 32°C, that is, to 10° to 12°C below the 42°C of normally regulated daytime body temperature. This readjustment of their body temperature set-point was sufficient to make their fat reserves last the night, despite vigorous shivering during most of the time they were sound asleep. Nevertheless, even with the caloric savings derived from hypothermia, the chickadees’ fat reserves in the morning were insufficient to last them through another day and night, such as could occur during a severe blizzard. To survive such commonly occurring emergencies or temperatures much lower than 0°C would require them to have special shelter at night where air temperatures are higher and convective cooling minimized and considerable energy would be saved (Buttemer et al. 1987).
Chickadees do not build winter sleeping shelters, but like other Paridae they show great flexibility in choice of roost sites for overnighting (Perrins 1976; Pitts 1976). Black-capped chickadees may sleep in almost any tight cranny or cavity (as can sometimes be deduced from their bent tail feathers in the morning); in dense vegetation such as vines; in conifers; and possibly in snow. Siberian tits have even been reported to dig 8-inch-long tunnels into the snow for overnighting (Zonov 1967). Although black-capped chickadees have not been reported to huddle at night or dig into snow, many of their relatives do (Smith 1991, p. 246).
Chickadee fluffing its feathers on a cold winter day. Tail feathers are bent from overnighting in cramped quarters to escape cold. (Drawn from photograph by David G. Aden.)
One of the chickadees’ remarkable winter adaptations is their plumage, which is denser than that of other birds their size (Chaplin 1982; Hill, Beaver, and Veghte 1980). Heat loss is mainly from the area around the eye and bill, and when the birds fluff out and then ball up to sleep, they are reducing specifically that area of heat loss by tucking their heads under their scapular (shoulder) feathers of the wing.
The fact that chickadees were cutting it close, though, even at modest winter air temperatures, only deepens the mystery of how golden-crowned kinglets survive. They are half the body size of chickadees and experience at some times double the temperature extremes of Chapin’s study birds. Do kinglets overnight without much larger fat reserves?
Charles R. Blem and John F. Pagels from Virginia Commonwealth University have provided the only data to help answer that question. In January–February 1983 they collected (shot) kinglets in Virginia throughout the day. Like the chickadees, the diminutive golden-crowned kinglets increased their fat stores during the day, from about 0.25 gram at 8 A.M. to about 0.60 gram at 5 P.M. Despite the kinglets weighing only half as much as the chickadees, these amounts of fat are nearly the same in absolute terms as the chickadees’. Thus, relative to body size, the kinglets put on twice as much fat per day as the chickadees. Nevertheless, even these fat reserves already seem low at modest air temperatures of 0°C for the northern winter nights of fifteen hours; Blem and Pagels calculated that a kinglet in such conditions