In the Field
Savings in a Snowbank
WITH THE LAST CLAMORING FLOCKS of geese disappearing into gray December skies, autumns great migration comes to an end. Woods and fields now belong to the winter birds, mostly small seed-eaters such as redpolls and siskins or insect-gleaners such as downy woodpeckers and nuthatches. Many of them will thrive because of reduced competition for resources, but a new hardship will be imposed when the snow comes, blanketing food and restricting foraging. One group of birds, however, specialists of a different sort, will fare better because of the snow. To the sedentary, mainly ground-dwelling grouse and ptarmigan, deep snow means good fortune.
Winters energy shortages pose a threat to all warm-bodied animals, whose survival comes down to a single, basic problem: how to balance their heat budget. As outside temperatures fall, the maintaining of a normal body temperature requires either a boost in energy intake, usually through increased foraging, or a reduction in heat loss. Of the two alternatives, the latter (which may involve a number of behavioral as well as physiological tricks) is often the first to be employed, because in terms of energy expenditure it is cheaper to cut losses than to try to find more food. This is where the grouse shine.
For most birds, options for trimming heat loss are limited. They add extra plumage during the autumn molt, as much as 30 percent; by fluffing feathers to further entrap air, they may boost insulation to perhaps 150 percent its summer level. But feathers alone seldom suffice where winters are extremely cold, and birds generally have not evolved compensatory behaviors such as huddling. On several occasions I have seen pygmy nuthatches flit, one after another, into a tree cavity or a hole in the side of a building until ten to twenty were packed in for the night. But only a handful of species are known to do this. Most birds simply shiver alone through the night, allowing their body temperature to drop slightly; the next day they frantically forage to replenish their energy reserves.
The gallinaceous (fowl-like) birds of the northern regions, however, have caught on to a trick no others use. Superbly insulated to begin with (largely through the addition of after-feathers, which grow from the main shaft of body feathers), grouse and ptarmigan tolerate cold better than most birds do, although they, too, eventually begin shivering for heat production. They also minimize their exposure more effectively than others by taking advantage of natures own winter insulationthe snowpack itself. Rather than roost in trees at night, grouse and ptarmigan may tunnel into the snow, sometimes diving from low tree branches and then "swimming" several inches forward until they are out of sight (only the blue grouse of the western U.S. mountains prefers to roost in dense conifers).
I have often observed ruffed grouse in the Northeast feeding hurriedly at dusk on the buds of aspen or maple, filling their crops with enough food to keep them going for many hours and then mysteriously vanishing in the dim light, just as it becomes too dark to follow them. Only in the morning, when I happen upon wing marks in the snow beside a birds exit hole, do I discover where it spent the night. Lucky is the skier who, after a nights snowfall has masked all signs, chances to pass near the roosting grouse, prompting the bird to explode from the snow.
To the grouse, the value of roosting in the snow is considerable. Burrow temperatures of captive grouse in Europe have been measured at 32 degrees F or slightly higher when outside air temperatures were below zero. Within the burrow the snow melts and refreezes, forming a cavity that traps warm air around the bird. This, coupled with a slight reduction in the birds body temperature, can effect a 45 percent savings in energy.
What makes roosting under the snow possible in the first place is yet another remarkable adaptation. Arguably the grouses greatest advantage in snow country is an ability to extract calories from a diet of coarse, fibrous materialsmostly buds, twigs, and conifer needles. Its winter fare may seem nutritionally narrow compared with summers eclectic offerings (seeds, greens, berries, caterpillars, various other insects, but the menu is at least extremely convenient, for even under the harshest conditions there will always be a ready source of food. And if needles and twigs are tough chewing, the gallinaceous birds have an answer to that too. Food is ground up in the birds gizzard (usually with the help of grit picked up along stream banks or roadsides) and passed along to the cecum, a fermentation chamber containing microorganisms that can break down cellulose and other recalcitrant material.
Prior to winter, both the crop and the cecum usually undergo enlargement (the cecum in the willow ptarmigan, a fifteen-inch bird, may reach a total length of forty inches), enabling grouse and ptarmigan to store and process more food over long periods of time and, if necessary, to spend two or three days under the snow during storms.
The black grouse of Finland, an extreme example, may spend as much as 95 percent of the wintervirtually all the time it is not gathering foodbeneath the snow. Others may vary in their habits, but to most grouse and ptarmigan, a "blanket" of snow takes on literal meaning.