The Forest Unseen_A Year's Watch in Nature

by David George Haskell

  The energy used by chickadees has been measured both in the laboratory and in free-living birds. On a winter day, the birds need up to sixty-five thousand joules of energy to keep themselves alive. Half this energy is used to shiver. These abstract measures become more understandable when they are converted into the currency of bird food. A spider the size of a comma on this page contains just one joule. A spider that fits within a capitalized letter holds one hundred joules. A word-sized beetle has two hundred and fifty joules. An oily sunflower seed has more than one thousand joules, but the mandala’s birds have no seed-filled bird feeder. Chickadees must daily find hundreds of food morsels to meet their energy budget. Yet the mandala’s larder looks utterly empty. I see no beetles, spiders, or food of any kind in the ice-blasted forest.

  Chickadees can coax sustenance out of the seemingly barren forest in part because of their outstanding eyesight. The retinas at the back of the chickadees’ eyes are lined with receptors that are two times more densely packed than are mine. The birds therefore have high visual acuity and can see details that my eyes cannot. Where I see a smooth twig, birds see a fractured, flaking contortion, pregnant with the possibility of hidden food. Many insects pass the winter ensconced inside tiny cracks on tree bark, and the chickadees’ discerning eyes uncover these insect hideaways. We can never fully experience the richness of this visual world, but peering through a magnifying lens gives us an approximation. Details that are normally invisible snap into view. Chickadees spend most of their winter days passing their superior eyes over the forest’s twigs, trunks, and leaf litter, sleuthing concealed food.

  Chickadee eyes also perceive more colors than mine can. I view the mandala with eyes that are equipped with three types of color receptor, giving me three primary colors and four main combinations of primary colors. Chickadees have an extra color receptor that detects ultraviolet light. This gives chickadees four primary colors and eleven main combinations, expanding the range of color vision beyond what humans can experience or even imagine. Bird color receptors are also equipped with tinted oil droplets that act as light filters, allowing only a narrow range of colors to stimulate each receptor. This increases the precision of color vision. We lack these filters, so even within the range of light visible to humans, birds are better able to discriminate subtle differences in color. Chickadees live in a hyperreality of color that is inaccessible to our dull eyes. Here in the mandala, they use these abilities to find food. Ultraviolet light reflects from the dried wild grapes that are sparsely scattered across the forest floor. Wings of beetles and moths are sometimes tinged with ultraviolet, as are some caterpillars. Even without the advantage of ultraviolet vision, insect camouflage is unmasked by slight imperfections detected by the birds’ precise perception of color.

  The visual abilities of birds and mammals differ because of events in the Jurassic, one hundred and fifty million years ago. At that time, the lineage that gave rise to modern birds split from the rest of the reptiles. These ancient birds inherited the four color receptors of their reptilian ancestors. Mammals also evolved from reptiles, splitting away earlier than the birds. But, unlike birds, our protomammal ancestors spent the Jurassic as nocturnal shrewlike creatures. Natural selection’s shortsighted utilitarianism had no use for sumptuous color in these night-dwelling animals. Two of the four color receptors that the mammals’ ancestors bequeathed to them were lost. To this day, most mammals have just two color receptors. Some primates, including those that gave rise to humans, later evolved a third.

  The chickadees’ acrobatic bodies let them put their vision to good use. A wing-flick takes a bird from one branch to another. Feet grasp a twig, then the bird falls, swinging from a branch tip. The beaks probe as the bird’s body pivots, still hanging, then wings flash open and the bird flits up to another small twig. No surface is left unexamined. The birds spend as much time upside down, peering under twigs, as they do upright.

  Despite the vigor of their search, the chickadees catch no prey while I watch. Chickadees, like most birds, give a distinctive backward flick of their heads as they swallow or, if they find a bigger morsel, will hold the food in their feet as they pound it with their beaks. The flock stays in my sight for just fifteen minutes, finding no food. The chickadees may need to call on their fat reserves to survive the cold. These reserves are essential to winter survival, and they allow chickadees to make good use of winter’s variability. When the weather warms, or when birds find a cluster of spiders or berries, the flush of food is turned into fat that carries the birds through times when the feeding is poor and the weather is cold.

  The degree of fatness varies among individual birds. Chickadees feed in socially stratified flocks, usually composed of a dominant pair and several subordinates. Dominant birds get access to whatever food the flock finds, so they generally eat well whatever the weather. These high-ranking birds have trim bodies. Subordinate chickadees bear the brunt of winter’s hardship, eating well only intermittently. The low-status birds, often youngsters or failed breeders, compensate for the variability of their food intake by getting fatter, buying insurance against lean times. But there is a cost to chickadee fatness. Rotund birds are easier prey for hawks. The fatness of each chickadee is a balance between the risk of starvation and the risk of predation.

  Chickadees supplement their fat stores by jabbing insects and seeds under flaking bark, storing food for later recovery. Carolina chickadees are particularly fond of caching food by poking it into the undersides of small branches. This habit may be a guard against thievery from less agile bird species. Nonetheless, caches are vulnerable to plunder, so each chickadee flock in the forest defends a winter territory from which neighbors are vigorously excluded. Non-caching chickadees in other parts of the world are much less territorial.

  Larger bird species often join chickadee flocks in the winter. Today, a downy woodpecker chisels for larvae in the bark of an oak tree, then flies after the chickadees when they flit east. A tufted titmouse also travels with the flock. The titmouse bounces among branches like the chickadees, but it is less agile, preferring to light on twigs without swinging from branch ends. All the birds call, keeping the flock together. The chickadees and titmouse chatter and whistle, the woodpecker gives high-pitched pik notes. This flocking behavior gives the group members safety from hawks, which are easier to spot when many eyes are vigilant. But chickadees pay a price for safety in the crowd. Tufted titmice are twice as heavy as chickadees, and the larger birds dominate, pushing the chickadees away from dead branches, higher twigs, and other preferred feeding locations. These subtle changes in location result in significant lost feeding opportunities for chickadees. In flocks where titmice are absent, chickadees are better fed. Survival in the winter mandala therefore requires not just sophisticated physiology but careful negotiation of social dynamics.

  Daylight is fading now. I move my chilled limbs and rub my ice-crusted eyes in preparation for the walk out of the forest. The birds will continue their search for food a few minutes longer, then they will head to their roosts. As light fails and the temperature drops, chickadees will gather in holes left by fallen tree limbs, sheltering from the wind’s heat-ripping power. The birds huddle in groups, giving a nod to Bergmann’s rule by creating a ball of birds with a large volume and a relatively small surface area. Then the chickadees’ body temperature will fall by ten degrees into an energy-saving hypothermic torpor. At night, as in the day, integrated behavioral and physiological adaptations give the birds an edge over winter. Torpor combined with huddling halves the chickadees’ nighttime energy needs.

  The chickadees’ adaptations to the cold are remarkable, but they are not always adequate. There will be fewer chickadees in the forest tomorrow. Winter’s chill hands will pull down many of these birds, dragging them deeper than the appalling emptiness I felt when I experienced the cold. Only half the chickadees that fed among the falling autumn leaves will live to see the oak buds open in the spring. Nights such as tonight cause most o
f the birds’ winter mortality.

  This week’s glacial temperatures will last just a few days, but the spike in bird mortality will change the forest in ways that extend throughout the year. Deaths on winter nights check the chickadee population, trimming any birds that exceed the scant supply of winter food. Carolina chickadees each require, on average, three or more hectares of forest to sustain themselves. This square meter of mandala therefore supports just a few hundred-thousandths of a chickadee. Tonight’s cold will remove any excess.

  When summer arrives, the mandala will be able to support many more birds. But because the abundance of resident species like chickadees is kept low by winter’s meager supplies, the food available in summer vastly exceeds the resident birds’ appetites. This great seasonal flush of food creates an opportunity that is exploited by migrant birds that risk long flights from Central and South America to feed on the excess in forests throughout North America. Winter’s cold is therefore responsible for the annual migration of millions of tanagers, warblers, and vireos.

  Overnight deaths will also fine-tune the chickadee species’ fit to its environment. Smaller Carolina chickadees will be more likely to perish than their bulkier kin, reinforcing Bergmann’s latitudinal pattern. Likewise, extreme cold will purge from the population those birds whose shivering abilities, feather fluffiness, or energy stores are deficient. In the morning, the chickadee population in this forest will be better matched to winter’s demands. This is natural selection’s paradox: from death comes life’s increasing perfection.

  My own physiological inadequacy in the cold also has its origins in natural selection. I am out of place at the icy mandala because my ancestors have dodged selection for cold-hardiness. Humans evolved from apes that lived for tens of millions of years in tropical Africa. Keeping cool was a much bigger challenge than keeping warm, so we have few bodily defenses against extreme cold. When my ancestors left Africa for northern Europe, they brought with them fire and clothes, carrying the tropics to the temperate and polar regions. This cleverness produced less suffering and fewer deaths, unquestionably good outcomes. But comfort sidestepped natural selection. We are condemned by our skill with fire and cloth to be forever out of place in the winter world.

  Darkness comes and I retreat toward my inheritance, the warm hearth, leaving the mandala to the avian masters of the cold. This mastery was earned the hard way, through thousands of generations of struggle. I wanted to experience the cold as the mandala’s animals do, but I now realize that this was impossible. My experiences come through a body that has taken a different evolutionary path from that of the chickadees, precluding any fully shared experience. Despite this, my nakedness in the cold wind has deepened my admiration for these others. Astonishment is the only proper response.

  January 30th—Winter Plants

  A continuous low-pitched roar comes from the wind raking the trees on the high bluff above the mandala. Unlike the northerly gales earlier in the week, this wind is from the south, and the bluff protects the mandala from all but a few eddies and gusts. The changing wind has eased the temperature. It is just a couple of degrees below freezing, warm enough to sit comfortably for an hour or more in winter clothes. The urgent, unrelenting physical pain of the cold is gone, and my body welcomes the benign air with a glow of quiet pleasure.

  Birds in a passing flock seem to revel in their release from the arctic death grip. Five species travel together: five tufted titmice, a couple of Carolina chickadees, one Carolina wren, a golden-crowned kinglet, and a red-bellied woodpecker. The flock seems bound by invisible elastic threads; when a single bird gets left behind or strays beyond the ten-meter radius of the flock, it is yanked back to the center. The whole flock is a rolling ball of agitation as it moves through the inanimate snowy forest.

  The titmice are the most vocal of the birds, streaming a continual jumble of sound. Each titmouse jabs high-pitched seet notes, creating an irregular beat against which play their other calls, hoarse whistles and squeaks. Some birds repeat pee-ta pee-ta, a sound that was absent from their repertoire earlier this week in the deep cold. This bright two-noted sound is the breeding song. Despite the snow, these birds are already turning their attention to spring. Egg laying will not happen for another couple of months, but the extended social negotiations of courtship have begun.

  The life-filled exuberance of the birds contrasts with the mandala’s plants. Their gray branches and bare twigs below offer a scene of desolation. Death juts from the snow: fallen, partly decayed maple branches and the frayed stubs of leafcup stems protrude, each stem ringed by a circle of sublimated snow that reveals the dark litter below. Winter appears to have delivered a thorough defeat.

  Yet life persists.

  Bare shrubs and trees are not the skeletons they appear to be. Every twig and trunk is wrapped by living tissue. Unlike birds that survive by fighting the cold with food won from winter’s tight fist, plants somehow endure without re-creating an internal summertime. The birds’ survival is astonishing, but the plants’ resurrection after a full surrender is so far removed from human experience that it edges into scandal. The dead, especially the frozen dead, should not return.

  But, return they do. Plants survive in the same way that a sword swallower survives—with careful preparation and meticulous attention to sharp edges. Plant physiology can generally cope with mere chilling. Unlike the chemical reactions that sustain humans, plant biochemistry can run at many different temperatures, and it does not fail when cooled. But when cooling turns to freezing, problems start. Expanding ice crystals will puncture, tear, and destroy the delicate inner architecture of cells. Plants in winter must swallow tens of thousands of blades, keeping each one away from their fragile hearts.

  Plants start their preparations several weeks ahead of the first freezes. They move DNA and other delicate structures to the centers of their cells, then wrap them in cushioning. The cells get fattier, and the chemical bonds in these fats change shape to make them fluid in cold temperatures. The membranes around cells become leaky and flexible. The transformed cells are padded and limber, able to absorb ice’s violence without harm.

  Preparations for winter take days or weeks to complete. A frost out of season will kill branches that, when properly acclimated, could stand the coldest nights of the year. Native plant species are seldom caught out by freezes; natural selection has taught them the seasonal rhythms of their homes. But exotic plants have no local knowledge and are often heavily pruned by winter.

  Cells not only change their physical structure but soak themselves in sugars, lowering the freezing point like salt scattered on icy roads. Sugaring happens only inside the cells, leaving water around the cells unsweetened. This asymmetry allows plants to exploit an expected gift from the laws of physics: heat is released when ice forms. Cells surrounded by freezing water receive a temperature boost of several degrees. During the first frosts of winter, the sugared insides of cells are protected by the unsugared water surrounding them. Farmers exploit this burst of warmth by misting their crops on frosty nights, adding another layer of heat-releasing water.

  Once all the water between cells has frozen solid, no more heat is released. But water inside cells is still liquid. This liquid oozes out of the leaky membrane around the cell. As the water leaks out, it leaves behind the sugars, which, being large molecules, cannot pass through the membrane. This process gradually draws water out of the cell as temperatures drop, increasing the inner concentration of sugars and further dropping the freezing point. When temperatures are very low, cells pucker into balls of syrup, unfrozen repositories of life, surrounded by shards of ice.

  The Christmas fern and the mosses in the mandala face an additional challenge. Although their evergreen leaves and stems let them feed on warm winter days, the source of their green, chlorophyll, can be unruly in cold weather. Chlorophyll captures energy from the sun and converts it into a buzz of excited electrons. In warm weather, the electrons’ energy is quickly shunted to the foo
d-making process in the cell. But this shunt seizes up in cold weather, leaving cells awash in overexcited electrons. Unchecked, the undirected energy will trash the cell. To forestall the riot of electrons, evergreens prepare for winter by stocking their cells with chemicals that intercept and neutralize the unwanted electron energy. We know these chemicals as vitamins, particularly vitamins C and E. Native Americans also knew this and chewed winter evergreens to keep healthy through the winter.

  Ice permeates the mandala’s plants, but each cell carefully recoils, enforcing a microscopic separation between ice and life. By reversing this cellular contraction, twigs, buds, and roots are able to revive in spring and carry on almost as if winter had not happened. A few plant species, however, take a different path. Leafcup herbs completed their short eighteen-month lives last fall and now stand dead, surrendered entirely to winter. They have sublimated into a new physical form, like snow passing into vapor. Like vapor, these new forms are invisible, but they surround me. Buried in the mandala’s litter are thousands of leafcup seeds, waiting out winter. Because seeds have hard coats and dry interiors, they pass through the cold months largely protected from the assaults of ice.

  The impression of desolation in the mandala is superficial. Within the bounds of this one square meter are hundreds of thousands of plant cells, each one wrapped into itself, intensified in its withdrawal. The quiet gray exterior of plants, like gunpowder, belies the energy that is latent here. So, although titmice and other birds give a vigorous display of life in January, they are trifles compared to the power stored in the quiescent plants. When spring sparks the mandala, the energy released will carry the whole forest, birds included, through another year.

  February 2nd—Footprints

  The tips of a maple-leaf viburnum have been chiseled off, leaving beveled stubs along the shrub’s branches. The animal that clipped these tender shoots has left three footprints in the mandala, aligned east to west. Two almond-shaped impressions make up each footprint, sunk two inches into the leaf litter. This is the signature of a cloven hoof, the seal of the artiodactyl clan. Like nearly every terrestrial community the world over, the mandala has been browsed by a cleft-hoofed mammal, in this case a white-tailed deer.