NESTS AND DENS - Winter World: The Ingenuity of Animal Survival - Bernd Heinrich

Winter World: The Ingenuity of Animal Survival - Bernd Heinrich (2003)


A large part of adapting to the winter world involves creating a suitable microclimate. Birds wear insulating feathers, mammals fur, and we wear adaptive clothing. But a number of animals, primarily beavers, bears, humans, birds, and some insects take a step beyond insulation, building nests or dens that supplement or take the place of body insulation.

In all animals, den construction is constrained by building materials, energy requirements for heating and cooling, defense, and accessibility. The Anasazi Indians of the southwestern United States built their homes high on inaccessible and defensible cliffs, choosing locations where an overhanging ledge offered shade at noontime in summer and exposure to direct sunshine in the winter when the sun is low. To the north and in Europe and Asia, when wood was not available, Ice Age hunters built huts framed with mammoth tusks and covered them with skins and sod; the early Eskimos did similarly, substituting whale bones for mammoth tusks. They of course invented the igloo, that marvel of simplicity and efficiency. Using hard-packed, fine-grained snow cut with a knife into blocks roughly twice as long as high, a man could build a house in less than an hour by spiraling the blocks upward, each slanted slightly inward.

In our own expansion into the winter world we at first relied on the retention of body heat, which is still the main source of heat in an igloo. Then we used fire as a supplement. The first use of fire for heating our hearth is attributed to European sites of about 500, 000 to 100, 000 years ago, when Ice Age hunters leaned poles against the cave entrance and covered them with hides. Not much changed in the way of heating (except for containing the fire in a fireplace) until Benjamin Franklin invented the Franklin stove, followed eventually by the invention of central heating. Instead of building nests, Ice Age people stayed warm and alive, much like a chickadee or a kinglet does in winter—by metabolically burning fat from the animals they killed.

Among possibly the first nests on earth, and still among the most impressively engineered ones, are those of insects who have been perfecting their building techniques for maybe 300 million years. Every autumn after the leaves fall in New England, I see the nests of one species of wasp, the white-faced hornet (Dolichovespula maculata), hanging conspicuously in the trees. Each nest is started in May by a single female that has hibernated through the winter. She uses her mandibles to scrape fiber from the dead wood of a branch, mixes it with her saliva for a papier-mâché slurry, and then makes tissue-thin strips of paper by adding one load at a time on the bottom edge of a slowly growing globe that will house her and eventually her entire brood of hundreds of eggs, larvae, pupae, and adult offspring. As her daughters come of age, they help their mother construct the nest, laying on layers of white, brown, and gray papers, differently hued depending on the wood. Despite its thinness, the paper does not dissolve in the rain. It remains durable all summer long and through the winter. As my experiments suggest, a good soaking seldom wets anything beyond the first layer of the paper nest.

White-faced hornet nest with two wasps adding paper to the bottom edge of the outermost sheet.

As the wasp family grows and more room is needed, the insects enlarge their nest by recycling the paper walls from the inside to make new, larger ones on the outside. A nest starts out in May no bigger than a walnut with just one paper shell, and ends up basketball-size by late summer having about a dozen layers of paper insulation surrounding almost as many horizontal combs with pupae and larvae, hanging one above the other inside. Each air layer between successive sheets of paper acts as insulation, and the temperatures inside the nest stay between 29° and 31°C even in cold weather down to 5°C, as the wasps shiver with their muscles powering their wings to maintain their own body temperature above 40°C. The heat loss from the body then heats the nest and young.

We are recognizably humans for only about 3 million years, but definitive birds have been flying well over 100 million years. At first their nests were, like those of their reptilian ancestors, simply depressions scraped in the ground or hiding places for their eggs and/or young. As birds became ever more metabolically active, in part to become creatures capable of rapid and sustained flight, they at the same time became warm-blooded and the ability to produce heat ended up being not just an option but a necessity both for them and their young. Expanding northward or into seasonal environments then generated selective pressure for their nests not only to be receptacles for their eggs and young, but also warm shelters.

The nests of northern and late-winter nesting birds, such as gray jays, raven, crossbills, and golden-crowned kinglets, are superbly insulated structures that are essential for these birds’ lives in their respective environments; the eggs and young must be kept warm, while minimizing heating costs.

The golden-crowned kinglet’s nests are built by suspending strands of spiderwebs, bark strips, and caterpillar silk into a hammocklike configuration within the hanging twigs of a spruce or fir. Moss, lichens, and strips of paper birch bark are then incorporated into the bottom and wall of the nest. The resulting cup-shaped structure is suspended into twigs by its lips and its walls. It is insulated with snowshoe hare down and with small bird feathers, numbering 2, 486, 2, 674, and 2, 672 feathers in three nests where individual feather counts were made (Thaler 1990, pp. 83-84). The nests are hidden from view from above, and partially protected from rain and sun by overhanging twigs of spruce or fir. However, regardless of how elaborate, almost all bird nests are built for only one-time use, and a good time to find many of these discarded nests is in the winter.

Bird nests are records of genetically encoded behaviors. Each bird performs unlearned nest-construction behavior, duplicating fairly precisely what its parents had done. Some nest-construction behaviors are relatively simple. Geese, for example, pick an appropriate nest location, and then simply lay their eggs there. Often the female pulls in nearby vegetation from all around and banks it against her body. She then sheds her belly feathers and tucks them under her for a nest lining. In the Canada geese I’ve watched in a bog by my house, this down-lining of the nest occurs only after or when the eggs are laid. (Reduction of the goose mother’s belly insulation could probably be hazardous when she swims in ice-cold water, but it may be necessary on the nest, so that body heat becomes available to the eggs she sits on.)

Most birds collect specific nest material from a wide area, and they execute specific behaviors both to find and work with it. Bicknell’s thrushes in the mountains of Maine and Vermont line their nests with the rhizomes of horsehair fungus. One year the pair of tree swallows in one of my birdboxes in Maine built its nest with grass from beneath the birdbox then lined the nest exclusively with white duck feathers. The nearest white (domestic) duck was at least three miles distant.

There is some flexibility in nest construction (but not much). A tree swallow at my house in Vermont built a nest almost exclusively of feathers (87 large white ones presumably from chickens, and one small black one from a raven, to be exact). House wrens used the same nest box immediately after the swallows left the eggs that had rotted (after they stopped incubating during a week of cold, early spring weather). But the wrens built a nest of 675 twigs that filled the box nearly to the top. This nest had no feather lining, although for their second successful brood that they raised in the same box that season, the wrens replaced the nest that I had removed with one made of only 40 twigs and lined with 185 pieces of grass and rootlets and 52 feathers of many kinds of birds.

Male ravens build their nest by holding large thick sticks they break from trees with their bills. With rapid head-bill shakes, they vibrate the sticks against or on the substrate of ledge or tree branches at the chosen nest site, until they anchor there. After the nest platform is built, the bird habitually perches in the center, where fewer sticks then become anchored, and a nest cup with a high rim develops. Once the basket of sticks is completed, both members of the pair then upholster it with bark, moss, grass, and animal fur. Almost all nests in New England contain deer hair, sometimes a dense felt of it. Snowshoe hare, bear, and moose hair are also commonly used to cushion the eggs and help insulate them. The female alone incubates for twenty-one days even at the typically subzero temperatures in February and March.

Robins build a symmetrical hard mud cup that is surrounded by loose debris and lined with soft grass. The whole process involves instinctive responses. Two nests I watched being made were each almost completed in two days. On the first dawn, the female robin deposited a haphazard pile of grass, twigs, and birch bark strips at the future nest site. At dawn the next day, she started bringing mud, making one trip after another, at one-to three-minute intervals for five and a half hours. During the first hour or so the male followed the female but did no work; then he perched in the nearby woods and sang quietly, while she worked. No work was done in the afternoon. By the third morning the female made only a few trips, and she was unaccompanied by her mate. Without exception after depositing the mud, she squatted down and vibrated her body for a second or two; then she got up, turned a few degrees to right or left, and repeated the squat-vibration. She performed as many as sixteen of these routines after dumping a single billful of mud or other debris. Her nest-shaping did not seem to be attributable to a conscious plan. On one long beam under a shed, I found where a robin had built a two-cup nest, apparently because she got started turning at two similarly appearing spots on the initial pile of debris that had spread too far laterally on the smooth beam. I found a phoebe who had built a similarly misconstructed nest also on a long even beam where it might have been difficult to fix the precise nest location. In real estate, location is everything, especially in birds.

Although among geese and robins the female alone builds the nest, in some species (principally wrens and weaverbirds) the nest is built primarily by the male. Such male-made nests initially serve as sexual attractants. The female chooses the nest and through it, the builder, and she signifies her choice by adding the nest lining according to her own finicky specifications. I recall watching African spotted weaverbird males in a colony, all dangling from their half-finished nests and beating their wings and calling to show off. When prospecting females entered a nest to inspect, the male owner went wild in frenzy. In the northern New England woods, perhaps one of the best examples of this is the winter wren, which builds its nest hidden deep under the roots of trees that have been blown over by the wind. The wren’s nest is a snug little cavity with walls camouflaged with a lattice of moss and conifer twiglets. It has a quarter-inch entrance hole at the front. A male wren may build several of these nests in an area (I found a sedge wren who built six in his territory), but only the one chosen by the female will be lined with fur and feathers. The “false nests” apparently not only give the female more choice but may also prevent unmated males from moving into the neighborhood.

Weaverbirds of many species literally weave hanging fiber bags that resemble our textiles. However, one species, the sociable weaver (Ploceus cucullatus) of Africa (a close relative of our local weaverbird, the common suburban English sparrow) builds communal nests that in size and shape superficially resemble human-built thatch-roofed huts. Generation after generation of birds add to these apartment houses until they may weigh several tons and fill out the entire top of a large tree. The apartment complex shields the individual nests within from the sun and from extreme temperature fluctuations in the bird’s desert environment of southern Africa where nights are commonly cold and days are commonly hot.

Aside from protection from the elements, an important ultimate or evolutionary reason for such astounding diversity of nests and nest locations is predation. By far the greatest vulnerability a bird experiences is often in the long egg and nestling stages. Nest predators include other birds, as well as snakes and mammals. Nest inaccessibility works as an adequate defense for some, but most species rely on hiding their nests to reduce predation. However, for effective hiding, much depends on what others are doing.

In the winter woods, much is buried and hidden, but more is revealed. Curtains of snowflakes drifted over my face, and the snow already piled into cushions muffled my footsteps. Two feet of fresh powder had already fallen on solidly frozen ground layered with fallen leaves. Recently those leaves had obscured the views through an otherwise opaque forest of green. Now, at the end of January, I could see through the forest from one large maple tree to another, to beech trees, root tip-ups, and beyond to an open bog bordered with viburnum thickets. I was looking for bird nests that had in the summer been almost impossible to see because they were then enveloped in leaves. They would now show up as dark silhouettes with white caps. (The ground nests obviously will be missed, but most are almost invisible even without a covering of snow.)

We often find nests by sheer luck. But like nest predators, we improve our chances if we know where and what to look for. That is, present success depends on previous success. Similarly, predators also develop search images based on their previous experience. But by focusing on the familiar nests they may miss those that are different or at other than the usual locations. Variety of nests and nest sites is often a key to nest survival in many species. A simple thought experiment shows why. Suppose every species of small songbird were to tuck its nest behind the loose dead flaking bark of dead balsam fir trees, as brown creepers do. If every bird species were to choose such a site, then it would not be a hiding place. Instead, these sites would soon indicate where to find eggs and tender nestlings, in the same way that the Golden Arches now show us where to find a hamburger. What applies to the nest site applies to the nest itself as well.

Red-eyed vireo nest in sugar maple in the forest understory.

Programming for precise nest construction yields species-specific nests, and the more different or exotic the nest appearances are for different species, the less any one would stand out to predators. Like a nest predator, during my winter walks I also hunt using search images for the nests with which I’m familiar. One of the first of these nests I looked for on this winter day was that of the red-eyed vireo. Each vireo nest is suspended on its upper rim in a horizontal fork of a thin branch, usually in the understory of a hardwood forest. Each nest looks like a light-colored round lump, a little like a hornet nest, but when examined from up close you see how the nest is suspended at its lips by what appear to be spiderwebs and strips of birch bark.

On this day, when I found and looked at the nest of the red-eyed vireo closely, I was surprised to discover pieces of gray paper from the nest of a white-faced hornet that had been incorporated into the outside nest wall, along with the always present birch bark and spiderwebs. The white-faced hornets’ communal nests, unlike birds’ nests, are added to and grow throughout the whole summer. By fall, however, when the worker wasps and drones die and the mated queens hibernate underground, the wasps’ nests are abandoned. Pieces of the nest can then be removed with impunity, but merely touching an occupied nest, as is well known and as I confirmed in experiments where I agitated these wasps, instantly results in an exit of hornet guards that attack all moving objects nearby and sting them mercilessly. I presumed therefore that the piece of hornet-nest paper that was incorporated in what seemed like an outside decoration must have originated from an old, abandoned hornet nest, since a hornet would not hesitate to attack and sting, and likely kill, a bird or other small animal that disturbed the communal nest.

I seldom find a hornet nest. The vireos must have traveled far to get their wasp-nest paper. Was this a typical red-eyed vireo nest? I remember the first red-eyed vireo nest I ever found. I was eleven years old, it was my first spring in Maine, and I had climbed the big sugar maple tree next to the road on the Adamses’ farm, where I spent an ecstatic summer. The tree was one that Floyd, the father of the family, had tapped in the spring for sugar sap. I do not, of course, recall if it had wasp-nest paper on the outside, but I wish I did. But being alerted by this nest, and the memory of it, I then made a special effort to examine all red-eyed vireo nests that I found in Vermont and Maine. All twelve that I found had at least some white-faced hornet paper on the outside. Since a vireo nest is globular and resembles a white-faced hornet nest in shape, it seems plausible that incorporating wasp-nest paper would enhance that effect and keep the premier woodland nest predators (squirrels, chipmunks, and blue jays) from coming close. Indeed, a slight jiggling of a branch with a wasp nest causes the inhabitants to attack en masse—a response no squirrel or jay would want to risk. Even if one of these common bird feeder chow-hogs is only slightly deterred by something as seemingly trivial as a nest decoration, that might still make a big difference in the long run. I estimate that chipmunks and red squirrels alone raid well over 60 percent of allsongbird nests in my woods.

Redstart nest in young sugar maple at the edge of a swamp.

The linings of the vireo nests were all of thin, firm fibrous strands, but the kinds chosen differed among the many nests. Long reddish dead white pine needles were used in two nests. Hairy-cap moss fruiting stalks were employed in another. Other materials included very thin grass stems, the dried stems of sugar maple flowers, fine strips of ash bark, the rachis from decaying fern fronds, and sedgelike fibers that I was unable to identify.

Northern oriole nest in elm.

As I left the hardwoods where I found the red-eyed vireo nest, I descended onto a beaver bog where the frozen pond surface was bordered with already long-yellowed dry sedges.

Patches of light brown cattails were giving way to spiraea bushes, and arrowwood thickets that then graded into alders and young maples. Red-starts and least flycatchers at the latter site wedged their nests into the vertical forks of young trees. Within the cattails I found a half-dozen red-winged blackbird nests and three common grackle nests. They were all within several inches of the ice in the wilted, now-dry cattail leaves. The long-discarded bulky nests of catbirds and cedar waxwings were still visible in tangles of thin twigs where in the summer they were almost totally hidden in the dense foliage. The catbird nests were built of long twigs and robust plant stems, but the almost invisible signature that distinguishes them from similar nests (blue jay, rose-breasted grosbeak) is strips of grape bark. The one cedar waxwing nest I found was, as is typical, decorated with green moss. The low twiggy spiraea bushes revealed the flimsy grass nests of chestnut-sided warblers and also a yellow warbler nest made of cattail seedfluff felted together into a deep fluffy cottony cup. Goldfinches had left their plant-down-upholstered nests in the more open-standing young trees and bushes.

An elm tree along the beaver bog edge held the almost perfectly preserved baglike nest of a northern oriole. Its weave of tough plant fibers that surely rival those of hemp (which I found in decaying milkweed stems) allowed it to be suspended in the wind-whipped tip of a long thin twig where nest-raiding squirrels seldom travel. Near ground level I also noticed a jagged hole through the thin, tough bark of a rotten gray birch stub. It was the entrance to the nest cavity excavated by a black-capped chickadee. All Paridae, or tits, the family to which chickadees belong, hide their eggs in enclosed spaces. Chickadees have small weak bills yet they commonly hammer out their own nest hole in soft rotting wood. They sometimes use preexisting cavities instead, such as those made by a woodpecker. Within them they build a soft nest of moss, hair, and feathers.

Catbird nest in willow with nightshade vine.

Building material often dictates architecture and gives new options for unique nest sites. Barn and cliff swallows, for example, can place their nest on inaccessible cliffs (usually in dense communities of dozens of nests), essentially constructing their own often reusable stucco birdhouses out of mud mortar that remind me of Anasazi cliff dwellings, making one wonder if Anasazi were inspired by cliff swallows. Inside the swallows’ reusable mini-birdhouses (which are generally considered the nests) they build their nests of grass and feathers. Because of the mortar exterior that can be glued onto any solid substrate, many swallows can nest as readily on a cliff as on a barn wall. In contrast, bank swallows dig holes into sand-banks and within them build their grass-and-feather nests at the end of sandy tunnels. Tree swallows use instead the old nest holes woodpeckers have hammered into trees. Chimney swifts make a shallow nest cup by using their saliva to glue one little twig after another in parallel, forming a thin bare shelf onto the chimney wall. Their relatives, the cave swiftlets in Asia, have dispensed with twigs and build their whole nest out of saliva. (The coagulated hardened bird spit is considered a dining delicacy, since it is an expensive Asian restaurant item.)

Chestnut-sided warbler nest in spiraea bushes.

Chestnut-sided warbler nest in spiraea, roofed over with Clematis seeds. With seventy-six cherry pits stored inside. A deer mouse larder.

As I continued my walk and looked around for fascinating nests in the woods and bog on this snowy day, I found nothing quite as exotic as swift or swallow nests. Instead I found several nests belonging to chestnut-sided warblers. As is typical for this species, the nests were small, round, cup-shaped structures built of fine grass and situated in low spiraea bushes. To my great surprise, however, I found one nest that was domed over and it had a small round entrance hole to the side, reminding me of a wren’s nest. But this nest was just another typical grass nest of a chestnut-sided warbler, with a superstructure of plant down that had been added later by some other animal. Inside were 76 black cherry pits. After this surprising find I looked more closely and found another similarly revamped domed-over nest. This one, however, belonged to an alder flycatcher, and it contained 483 milkweed seeds, each with its fluffy parasol completely removed. I also found a goldfinch nest filled to the brim with about 1, 200 unidentified small black seeds, 254 milkweed seeds, and 1 sunflower seed. (I planted some of the small black seeds and they grew into commonroadside ragweed, whose pollen is a common allergen.)

Goldfinch nest in red maple sapling.

The three bird nests had, I suspect, been taken over by deer mice. Unlike other mouse and squirrel nests that serve as nurseries for young in the spring and as shelters from the winter elements for adults, these partially arboreal mice had cleverly rebuilt nests to serve as grain bins for winter food larders. Bird nests are often passed on among other bird species. For example, wood ducks, buffleheads, and mergansers depend on the old nest holes made by pileated and formerly ivory-billed woodpeckers. By providing safe nesting sites, woodpeckers are thus keystone organisms for a vast assemblage of birds the world over, including many owls, parrots, parids, flycatchers. But the recycling of the bird nests for food storage by the mice was, as far as I know, a previously undescribed behavior.

Cedar waxwing nest in arrowwood.

The mice made me wonder why almost all the other bird nests I found were destined to be abandoned. Why don’t birds reuse their nests for winter shelter? Even the nonmigrants that stay all winter such as chickadees, goldfinches, purple finches, and blue jays don’t reuse their nests or build new ones for shelter or for food storage. Yet most birds are skilled and eager builders. They construct amazingly elegant and functional nests with which to attract mates and in which to incubate their eggs and rear their young, but would they build even a crude shelter from the killing cold? Nothing doing! Well, almost nothing doing. There are, as almost always in the winter world, blatant exceptions.

The verdin (Auriparus flaviceps) is one. This mouse-gray bird which is halfway in size between a kinglet and a chickadee is a year-round resident of the southwestern United States and northern Mexico, where temperatures in winter can drop easily to 0°C at dawn. Not only is the verdin an apparent exception to most birds, it is a conspicuous exception because it builds numerous bedroom nests at anytime of year.

The verdin’s breeding and roosting (or bedroom) nests are both globular structures of about 20 centimeters in diameter. They are shaped like a small squirrel nest, but have only one entrance. Breeding nests take about six days to construct, while roosting nests are finished in just four days. Verdin nests are elaborate structures of three layers. The outermost layer is made of tightly interlaced thorny twigs that solidly anchor the nest into the tree; typically a mesquite, smoke tree, paloverde, or catclaw acacia. The middle layer is a compact mass of matted leaves, feathers, lichen, moss, and other various materials. Finally, the nest interior, especially in winter nests, is lined with soft fluffy material such as feathers from various birds, plant down, and spiderwebs. The winter roosting nests are larger and better insulated than summer nests.

Verdin nests are not only used by verdins. Glenn E. Walsberg, at Arizona State University, discovered a verdin nest in a shrubby desert wash that, during December 1989, was being used as a communal roosting place for fifteen to sixteen black-tailed gnatcatchers (Polioptila melanura). For five nights Walsberg observed these birds as they dispersed during the day, arriving back at the nest in the evening. Walsberg again counted them as they left the nest the next dawn, but this time the nest remained unoccupied for two nights. Obviously, the birds had alternate roosting sites and were traveling en masse from site to site.

Walsberg recorded internal nest temperatures near 30°C at night, even as air temperatures outside the nest dipped to near 0°C at dawn. Overnighting in the communally elevated nest temperature of 30°C would have cost each bird about 2.5 times less energy per night to keep warm. However, total energy savings were even greater because the birds were not only taking advantage of increased air temperature surrounding themselves, but also heating by direct body contact. I could not in the Maine woods, expect to discover anything resembling Walsberg’s amazing findings of gnatcatchers using a verdin’s nest, but I wondered why not. Surely a kinglet could use similar shelter.

By late afternoon the snow was coming down so thick that it almost blinded me. I passed the old hemlock tree with a honeybee nest that I’d found by lining bees the previous fall. A hollow ash tree where I had once seen a raccoon hole up in January stood near, but hearing the loud kek-kek-kek of a pileated woodpecker, I veered toward a huge dead elm instead. These woodpeckers had for several years in succession carved out their nest cavities in this tree. I saw three cavities, one above the other and spaced about four to five feet apart. Woodpeckers go to great trouble and effort to always make a new nest cavity each spring. There is then a yearly progression of fresh empty apartments for flying squirrels, crested flycatchers, barred owls, and potentially bats.

When I reached the woodpecker-made nest holes, I again heard the pileated’s kek-kek-kek-kek, reverberating through the woods, and then I saw the big gorgeous black bird. White wing-bars flashing, it sliced through the storm and landed on the far side of a sugar maple near me. It jerked its white-striped head with flaming red crest toward me from behind the shielding tree. I backed off, and the bird then flew to the elm and entered one of the old nest holes, possibly one of its own used for nesting in a previous spring.

Woodpecker holes are used by other birds for shelter in the winter as well. I have often found red-breasted nuthatches overnighting during the winter in old downy or hairy woodpecker holes, and the pygmy nuthatch (Sitta pygmaea), which often travels in small flocks, overnights during the winter in tree holes with its many companions for added warmth (Knorr 1957; Guntert, Hay, and Balda 1988), as do Carolina chickadees (Pitts 1976) and occasionally also bluebirds (Frazier and Nolan 1959).

It is one thing to find existing shelter and make use of it; it is quite another to expend time and energy making a shelter for future use. Surprisingly, some other northern woodpeckers do just that. In late November and early December when temperatures are dropping rapidly and the first snowstorms blanket the woods, I often hear steady tapping unlike a woodpecker’s more intermittent rapping for food excavation. Following the sounds to a decaying tree or thick tree limb, I find the ground and/or snow below littered with small light-colored wood chips. The head of a downy or a hairy woodpecker invariably appears at a round hole, then shakes to release a billful of shavings, and quickly tucks back in to resume hammering. At first I thought these birds were confused, perhaps by misreading the photoperiod—the hours of daylight versus dark that many animals use to keep track of the seasons—to begin nesting a half year early. However, the finished holes excavated in the late fall or winter (in more decayed wood than nest holes) were invariably used by the birds that made them for overnighting sites; I flushed out the bird in the evening by tapping the tree, but it quickly reentered the hole to spend the night there. (Woodpeckers use no nesting material beyond a layer of dry wood chips, but the nest cavities provide obvious protection from the cold.)

Since it may take a pileated woodpecker or a sapsucker up to two weeks to hammer out a cavity in a tree whose interior is softened by fungus, one wonders why other winter birds don’t build “regular” nests for shelter since such nests can be built in several days. For example, a pair of golden-crowned kinglets constructs its well-insulated nest of moss held together by spiderwebs in just four to six days (Ingold and Galati 1997). Might golden-crowned kinglets make winter nests as a solution to overnighting at subzero temperatures; or could they use nests made by other species?

I’ve never seen a kinglet carrying moss or any other nesting material in winter, and I think it is unlikely that these birds build winter nests for shelter. But if they don’t build winter nests, then why not? Do they not have the time because they are living too close to the edge? I have never seen a kinglet take even a one-second break from winter foraging. Maybe when their priority is to survive the day, then making an investment that doesn’t pay off until a later date makes no sense. Additionally, if the bird has to keep moving fast and continuously all day in an environment where food is patchy, then at the end of the day (when it may have traveled far), it may not have either the time or energy to relocate its nest. Having found an area of good food, it may be the better bet for a kinglet to stay put and forage until the last moment of daylight, and in fact that is what kinglets do. Finally, a winter nest would have to be designed differently than the open cup nests for rearing young. Although modifications of nest structure are possible, a new nest design might be too large an evolutionary hurdle for them to surmount.

The verdin’s and the woodpecker’s winter shelter nests are only slight modifications of their breeding nests. Their nests evolved first as receptacles for rearing young, and that same design then fortuitously made them preadapted for use as winter shelter later on. The nest of a finch or kinglet, in contrast, is cup-shaped. It is not preadapted for shelter in winter. Shelter nests require a cover. Cup nests in winter would quickly fill with snow.

There is no law that says a priori that a bird can’t switch from making one type of nest in one season to a different type of nest in another season. It is just one of those things that is unlikely, because the more specialized the programming is in one direction the more it precludes a new specialization in another direction.

Some effective shelters that birds make in the winter are much easier to construct than nests. The grouse that dives down into the deep fluffy insulating snow to escape the cold also modifies the snow by creating a cave at the end of its tunnel where it comes to rest. This shelter is for one-time use only. The bird may spend a night or a day or more in its shelter (depending on the weather), but it never returns to it. The next night it makes another shelter. However, if snow is absent, the grouse roosts in the cover of a dense coniferous tree instead.

As I was searching in the bog for the discarded summer nests of birds, I almost took for granted the most obvious winter shelter of all, the lodges of the resident muskrats and beavers. Beavers (Chapter 12) build lodges at the edge or directly in the pond, by piling up logs and sticks, mud, and sod into a steeply conical heap of five to seven feet above water level, and then excavating from underneath. A suitable lodge can be built in the fall, but old lodges are also refurbished and may be used by generations of beavers. I measured a lodge built the previous fall, and it reached seven feet above water (or ice) level and it was fifty-two feet in circumference. On this particular lodge almost all the sticks were light yellow, because the beavers had only recently eaten off the bark. Mud had been heaped onto the sides (but not the tip top) of the mound compacting and cementing the lodge together. In the winter when the mud freezes solid, the communal beaver lodge becomes practically impregnable to wolves and other potential predators.

The entrance to a beaver’s lodge is through a water lock at the bottom so that the beavers have a safe den site even in the summer. This was not lost on John Colter, the famous Yellowstone explorer who in 1809 escaped a band of pursuing Blackfoot warriors by diving into a beaver pond and hiding in the animals’ lodge. One parched summer a year or two ago, when the local beaver ponds had dried up and the water seal to a beaver’s den was gone, I also entered one of their dwellings. I carried a flashlight, to have a look around. The den I examined contained two platforms. One was slightly above the other, and in both the floors were liberally strewn with small debarked twigs, the remnants of take-home meals. (There were no remnants of fecal matter.) The chambers were roomy enough for me to be able to turn around, but it must have been a tight squeeze for a mated pair of beavers, together weighing about one hundred pounds. Add their two to five yearling offspring, and an equal number of young from the current year, and it must have been a tight, cozy fit. The space seemed awfully small for a crowd of perhaps a dozen beavers to spend half a year there in total darkness. Beavers do not hibernate and must therefore be quite tolerant of one another, as they would almost literally be rubbing against each other all winter. By spring, however, the yearlings get the wanderlust—or the cold shoulder as the parents evict them. After the ice melts off their ponds, I find these yearlings as common roadkill on highways that cross brooks or streams.

Even larger animals may build shelters. Usually before snow starts to cover the ground, a bear (or a family of a sow and her previous yearling cubs) chooses a den site for hibernation. Grizzlies dig tunnels into a hillside and excavate a cavity at the end. Black bears excavate depressions under a pile of brush, or the roots of a fallen tree, or use a rock cave or a hollow tree, and sometimes make or use no cave at all. The typical den cavity, if there is one, is about five feet wide and two or three feet high and upholstered with leaves, grass, and other debris raked in from the surroundings. Cedar bark and conifer twigs may also be bitten and torn from nearby trees and carried in. Yearling cubs help their mother build the den that they share with their mother. A bear’s den retards convective cooling by the wind, but temperatures inside the den are not much different from air temperature outside; bears rely mainly on their fur for insulation, which doubles its insulative capacity in winter. Once settled in, they don’t feed for seven months, living off their body fat until late spring. In January, sows give birth to two to three naked cubs, which don’t hibernate. They snuggle up to their mother and suckle for the three additional months that she hibernates all the while losing about twice as much body weight as males. Both sexes, however, stand a 99 percent chance of surviving the winter. The den stays clean because the sow does not urinate or defecate all winter. The cubs do, but despite a greatly depressed appetite the mother eats their feces.

The biologist “Bearman” Lynn Rogers, studying black bears in northwestern Minnesota in the 1970s, routinely visited them in their dens. “Most of those I visited in dens were wakeful enough that they lifted their heads and looked at me,” he wrote. “Although, in general, they seemed less sensitive to danger than they had been in summer, some were moderately aggressive…. Some did not wake even after gentle prodding and jostling.” And: “In one case, on March 27, 1970, I accidentally fell on a six-year-old female in her den. She didn’t wake up for at least eight minutes, even though her cub bawled loudly and I began prodding her. On January 8, 1972, I tried to hear the heartbeat of a soundly sleeping five-year-old female by pressing my ear against her chest. I could hear nothing. After about two minutes, though, I suddenly heard a strong, rapid heartbeat. The bear was waking up. Within a few seconds she lifted her head as I tried to squeeze backward through the den entrance. Outside, I could still hear the heartbeat, which I timed (after checking to make sure it wasn’t my own) at approximately 175 beats per minute.” A bear’s heart rate during the day is normally 50 to 90 beats per minute, although the heart rate of a sleeping bear in winter may decline to as low as 8 beats per minute. I wonder who was the more surprised by the visit: the researcher or the bear.

Other than biologists, few humans have entered bears’ dens to find out how cozy they might be. But in one case I am intimately acquainted with, a rabbit-hunting beagle wandered under a brush pile in the Maine woods, which happened to be the den of a black bear with her two cubs. The beagle attempted to retreat, but every time the dog tried to crawl out, the bear dragged it back inside. The sow acted as though the beagle was one of her cubs. The owner finally retrieved his dog unharmed, but only after the bear was tranquilized with a dart gun and the dog had then been denned for two days longer than it intended.

The second inadvertent entry into a bear’s den that I heard about involved a man. And it ended with a hastier exit. The man apparently broke through the snow and fell into a cavern on Ellesmere Island that happened to be the den of a polar bear with cubs. The heavily padded man (lucky for him, he was wearing his microclimate!) was quickly and summarily heaved back out with one swat of the bear’s great paw.

In the woods on the hills near my cabin in Maine, I commonly see what are popularly called bear “nests” up in beech trees. These I have entered, or examined closely, without being molested. They are platforms of branches that the bears pull toward themselves from all around to strip off the beechnuts when the trees still have their leaves. Leaf-shedding by trees is an active process in preparation for winter (to reduce ice-and snow-loading), and a broken and killed branch does not shed its leaves because the tree’s physiology is disrupted. As a consequence, where bears have been foraging for still unripe beechnuts in late summer one sees in November what superficially look like giant squirrel dreys (a name for squirrel nests) with many dead leaves. Structurally, the bear dreys are almost identical to chimp sleeping nests, the best constructions that any of our closest living relatives are able to produce.