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By Root 1255 0

the shift to nocturnal activity is costly from the perspective of energy supplies required to keep warm. Energy is saved by gliding, but the ability to glide precludes the laying up of fat stores, such as practiced by their relatives, woodchuck and other ground squirrels, that may get to be obese by fall. Also unlike ground squirrels, northern flying squirrels do not avail themselves of the huge potential energy savings of torpor, since they also don’t have a buffer of energy stores in body fat or food caches, nor change into a more insulating winter coat. As regards energy balance in winter, much seems to be stacked against them. One wonders what solution they might have that counterbalances their numerous presumed shortcomings for winter survival.

I am on the lookout for squirrel nests in my search for overnighting sites of kinglets, so I habitually bang on any tree that holds a nest to see if a kinglet seeking shelter might fly out. All the northern flying squirrel nests I’ve found were in dense spruce-fir thickets. I’ve never chased out a kinglet, but on occasion I’ve been rewarded with seeing one or two individual flying squirrels pop out of a nest, glide off the nest tree, and land on a neighboring tree. Assuming that the squirrels spend half or more of their time in winter in their nests, nest insulation should be of great relevance to energy balance. One nest that I examined in December 2000 was an unfinished framework of dry spruce twigs that contained no lining. Confined between several upward-bending branches, it had probably been abandoned before being finished because the space was too small. It showed, however, that the squirrel starts its nest structure by first making a globe of dry twigs, then inserts the lining. That December I found six other nests that had the same magpielike frames of small dry twigs but that did however contain the nest proper. (One had been torn open, and nest lining had been pulled out.)

Flying squirrel nest, covered by cushions of snow.

The nest linings varied from nest to nest. In one I found a mixture of moss, lichens, grass, and shredded birch bark. In two the lining was almost exclusively finely shredded birch bark. In a fourth it was almost all moss. In a fifth it was exclusively shredded cedar bark, and in the sixth the lining was in two distinct layers of shredded birch and cedar bark. (Many cedar trees in these woods show evidence of some of the outer bark having been stripped off, presumably collected by squirrels although bears also collect cedar bark.) When thoroughly dried this last football-size nest weighed 17 ounces, 12 ounces of which was lining, with a thick 8-ounce shell of densely packed usnea (“old man’s beard”) lichen and a 4-ounce layer of soft, finely shredded cedar bark within that. A good choice—the Northwest Indians used such shredded cedar bark to diaper their babies.

Even after heavy rainstorms the insides of the nests remained dry. Normally in winter these nests are also insulated on top when they are roofed-over with cushions of snow. All the nests had two entrances, one each on opposite sides. These entrances were not visible. They were, like the elastic ends of our mittens and socks, closed. Thus, in structure, each nest was like an old-fashioned hand muff. (In none of these, nor in seven additional red squirrel nests, was there one speck of bird feces, making it unlikely that they serve as kinglet overnighting sites.)

To get a rough idea of whether the flying squirrel’s nest indeed affords much insulation, I heated a potato to simulate the body of a squirrel and examined its cooling rates. At an air temperature of -13°C, a hot potato (60°C) cooled to only 42°C in thirty-five minutes when within the nest, and to 15°C in the same time period when outside it. My rough experiment only says that the nest indeed affords effective insulation. Of course the value of insulation would be much greater in wind, and it would be even more effective in a snow-covered nest. Furthermore, a squirrel, with its downy fur and a bushy tail wrapped around itself,