The Forest Unseen: A Year's Watch in Nature (28 page)

The balance between weight and power is the background to the rest of bird biology. Land-bound animals carry their reproductive organs around all year, but birds atrophy their testes and ovaries after they breed, shrinking them to tiny specks of tissue. Teeth are likewise dispensed with in favor of a paper-thin beak and grinding stomach. The droppings on our car windshields are another part of the birds’ strategy. By excreting white crystals of uric acid instead of watery urea, birds forgo the need for a heavy bladder.

Bird bodies are only partly solid. Much of the body is filled with air sacs, and many of the bones are hollow. These tubular bones provided humans with an unexpected gift. Archaeologists in China have uncovered nine-thousand-year-old flutes made from the wing bones of red-crowned cranes. The flute maker bored holes into the bone, creating a scale similar to the modern Western “do, re, mi.” Neolithic artists were therefore able to turn the magic of flight into another wind-borne delight.

The bubble-wrap lightness of the hawk is given an extra upward boost by the physiology of her thick pectoral muscles. Because bird
bodies run hot, at temperatures above forty degrees Celsius, the molecules that make up muscles react with speed and vigor, doubling the strength of their muscular contractions compared to our languid mammalian squeezes. Bird muscles are netted with capillaries that carry blood from a heart that is proportionally double the size of a mammalian heart and is far more efficient than the leaky pump that the birds’ ancestors, the reptiles, possessed. Blood is kept oxygenated by the bird’s unique one-way lung that uses the air sacs in the rest of the body as bellows to keep air streaming across the lung’s wet surface.

All this impressive physiology produces more than mere flight. The hawk dances on air. In just ten seconds, she stopped a rapid dive, rose vertically while turning, swept in a new direction, flapped upward, and curved into a rising arc, ending with a stall that parked her feet directly over a maple branch. The precision and beauty of bird flight is so familiar that our wonder is jaded. We should be frozen in amazement at the cardinal landing on the feeder or the sparrow banking around cars in the parking lot. Instead, we walk by as if an animal pirouetting on air were unremarkable, even mundane. The hawk’s dramatic rise over the mandala’s center jolts me out of this dullness, pulling away the blinding layers of familiarity.

Because the wing bones of birds are built on the same design as our forearms, we can imagine, at least in part, the lifting and folding of bird wings. But feathers add an alien layer of refinement, one that eludes our intuitive understanding. Hair is the closest human analog, but our simple protein ropes are slack and lifeless compared to the intricacy and control of bird feathers. Each feather is a fan of interlocking blades, organized around a central support, the rachis. This rachis is anchored in the skin by a cluster of muscles that the bird uses to tweak each feather’s position. The wing is therefore a coordinated collection of smaller wings, giving the bird the supreme control that commands our admiration.

As the hawk moves through the forest, the feathers deflect air down, pushing the wing upward. Air also flows faster over the down-curved
upper surface of the wing than it does over the concave underside. Fast-flowing air exerts less pressure, so the bird gets another lift. To land or change direction rapidly, she holds her wings at a sharp angle, breaking the smooth flow of air. The turbulence behind the wing acts as a brake, sucking the wing backward. The hawk’s command of this stall is so refined that alighting motionless on a twig seems easy.

The hawk in the mandala was hunting. Sharp-shinned hawks feed mostly on small birds, such as winter wrens, and the hawk’s wide, short wings let her slip between branches and accelerate powerfully as she pursues her prey. She uses her long tail to rudder through the tangled forest and snap upward, snatching flying birds from below with sicklelike talons. Any prey that escapes into tree holes or thickets is extracted with her gangly legs.

The hawk’s design has one drawback. Rounded wings create turbulence at their blunt tips, spilling messy swirls of air. These swirls drag on the bird, making sustained flight more expensive for sharp-shinned hawks than it is for falcons and other angular-winged species. In addition, the wings of sharp-shinned hawks are not fanlike enough to allow them to soar like vultures. This is a bird of the forest, at home darting through pine and oak branches, and her design is ill suited to long flights. Sharp-shinned hawks cover long distances by alternating flapping with short glides, using a compromise between the continuous flapping of a falcon and the easy drifting of a vulture. This is tiresome work, and the hawks must stop to feed and rest along their way, unlike more accomplished long-distance flyers.

Sharp-shinned hawks in Tennessee do not migrate, but they are joined by sharp-shinned hawks retreating from winter farther north. This autumnal flow of southbound sharp-shinned hawks has dwindled in recent years. Scientists first suspected that pollution or habitat loss was causing the falling numbers of migrating hawks. But this is apparently not the case. Instead, more sharp-shinned hawks are choosing to stay in the frozen northern forests rather than head south for the winter. These lingering hawks survive by loitering around human settlements,
making use of a remarkable new arrangement in the ecology of North America: the backyard bird feeder.

Our love of birds has created a new migration. This novelty is a west-to-east migration of plants, not a north-to-south migration of birds. The productivity of thousands of acres of former prairie land is shipped eastward, locked in millions of tons of sunflower seeds. These dense stores of energy are trickled from wooden boxes and glass tubes, adding a steady, stationary source of food to the otherwise unpredictably shifting winter food supply of songbirds in the eastern forest. Sharp-shinned hawks are therefore provided with a dependable meat locker, turning the forest into a home for the winter. Bird feeders not only augment the forest’s larder but, more important, they gather songbirds into clusters that make convenient feeding stations for hawks.

The expression of our yearning for the beauty of birds sets off waves that circle outward, washing over prairies and forests, lapping onto the mandala. Fewer migrant hawks from the north make life a little easier for the hawk in the mandala. Winter becomes less dangerous for songbirds also, perhaps edging up winter wren populations. More abundant wrens may nudge down ant or spider populations, sending an eddy out into the plant community when the spring ephemeral flowers offer their seeds to be dispersed by ants, or into the fungus community when a dip in spider numbers increases fungus gnat populations.

We cannot move without vibrating the waters, sending into the world the consequences of our desires. The hawk embodies these spreading waves, and the marvel of its flight startles us into paying attention. Our embeddedness is given a magnificent, tangible form: here is our evolutionary kinship splayed out in the fanning wing; here is a solid, physical link to the north woods and the prairies; here is the brutality and elegance of the food web sailing across the forest.

T
he branches over the mandala are entirely bare. They fragment my view of the clear sky with a tracery of dark lines. Directly above me, a squirrel balances on impossibly thin twigs at the top of a maple tree. The squirrel’s back feet grasp a twig while its forelegs and mouth reach out for clusters of seeds that have not yet fallen. Seed husks and small twigs rain in the animal’s wake, pelting the ground with their hard falls. Whole seeds also drift down, spinning slowly in the breeze and landing several meters west of the mandala. This is the first time in several weeks that I have seen a squirrel in the maple tree. Lately the hickory’s big, fatty nuts offered a better reward, but the nuts are gone and the squirrel has moved on to less favored foods.

One of the larger casualties of the squirrel’s destructive foraging lies in front of me. The maple twig is half the length of my forearm, and its tip is branched into several clusters of empty seed stalks. At first, my eye passes over it lightly, unconsciously dismissing it. Then my glance backtracks, and detail explodes. Fungi have yet to blur the inscriptions on the twig’s bark, so the story of this tiny piece of the canopy is writ clear.

The tan skin of the twig is scattered with creamy mouths, each oriented with lips opening parallel to the twig’s length. These are lenticels, just visible to the naked eye, through which air flows to the cells below. As the twig matures to a branch, then a trunk, lenticels become less numerous and are hidden at the base of cracks in the bark. Younger
twigs require a high density of lenticels to support their active, growing cells, just as a child’s lungs are larger in proportion to her body than are those of an adult.

Larger swollen crescents rise from the surface where leaves formerly sprouted. Each leaf scar is topped by a small bud, or a circular indentation where the bud once grew. Twigs will grow from these buds, then most of the twigs will die within a year, a seemingly wasteful way to grow. After several years, only one or two twigs from hundreds remain, thickened into branches. This extravagance is a general theme of life’s economy. Our nervous system also develops by ramifying into a complex web, then dying back to a simpler mature state. Social interactions do the same. The constant bickering among the members of a newly formed bird flock soon resolves itself into a simpler hierarchy where birds squabble only with their immediate superiors and subordinates.

Trees, nerves, and social networks are all systems that grow in unpredictable conditions. It is not possible for a maple seedling to know where the light will be strongest, or a nerve network to know what it will be called upon to learn, or a chick to know where it will fit in the social order. So, trees, nerves, and social hierarchies try dozens or hundreds of variants and pick out the best, molding themselves to their environments. Competition for light determines which twigs will live and die, and the varied architecture of trees grows out of the particularities of these hundreds of small events. A tree grown in the expansive light of an open field has a fan of branches that starts low on the trunk and gives the tree a wide, rounded profile. Here in the mandala, trees have few low branches and tight, cylindrical crowns, the result of crowding and competition for light. This process is analogous to evolution by natural selection, whereby a few winning characters are picked from the thousands of variants in each species. The process is already visible in the short length of twig in front of me. The older portion is bare, having already shed all its side branches, whereas the tip forks in a bush of curved matchsticks.

The smooth skin of the twig is interrupted by clusters of fine bracelets. These rings are the scars left by bud scales, the scooplike coverings that protect dormant buds through the winter. The tree’s efforts to shield its growing tips etch a record of the passage of time, leaving a yearly ring of scars. The distance between the rings tells the vigor of that season’s growth. Counting back from the tip: the maple twig grew an inch this year, an inch last year, and three inches in the two previous years. The oldest section was snapped by the squirrel’s feet, but the remaining part shows six inches of growth. This twig has been slowing its growth for the past five years.

I turn my attention from the maple twig to the bud scales of the saplings in the mandala. Do they tell the same story as the maple twig? The green ash that sprouts knee-high from the mandala’s center is topped by a magnificent bud, a swollen crown made of two large lobes flanked by two smaller teardrops. The bud scales that enclose this fat marvel are granular and the color of brown sugar. The marks from last year’s scales sit just an inch below—not much growth this year. Last year was little better, but the year before that resulted in two inches of growth, and the four-year-old wood is very long, eight inches. Could the weather of the last two years have been unwelcoming in some way?

A maple sapling on the west side of the mandala shows the same pattern as the maple twig and the ash, although the difference among the years is less marked. The growth patterns of a maple and an ash two feet north break the pattern. Their twigs have grown more than ten inches for two years. These trees have been thriving, particularly on the branches that face east. Something more complicated than a uniform response to weather is affecting the trees’ growth.

The variability in growth is partly caused by competition for light among the young trees. The collapsing growth rate of the ash in the mandala may result from the lush growth of the older ashes and maples that surround it. Four years ago, these older trees had not yet grown high enough to cast shade over the mandala’s center. For the
past three years they have progressively cast more shade, starving the ash.

The plants’ growth is also affected by events beyond the local stem-to-stem race for light. There is a large hole in the forest canopy just to the east of the mandala. Two or three years ago an old shagbark hickory fell, dragging with it several smaller trees. I did not see this particular hickory crash down, but I have seen others fall. Tree falls start with the sound of rifle shots as wood snaps and the trunk fails. Loud hissing follows as thousands of leaves are dragged through the canopy, the sound rising as the tree accelerates. The impact of the trunk is like a huge bass drum, felt as much as heard. A wave of smell follows. Torn leaves give a sickly sweet odor that mingles with the bitter, wet smell of rent wood and bark. If the tree’s roots were levered up by an unsnapped trunk, the ground is gouged and the root-ball stands up to six feet tall. The mess is impressive—smaller trees flattened, vines pulled from the canopy, twisted limbs everywhere. Once they are down, we can see what huge organisms trees are, like beached whales. A big tree fall can rip up a patch of forest the size of several houses, especially if it pulls other trees with it.

After the tree falls, light rushes in. Saplings that were not crushed or suffocated by fallen wood are bathed in light and grow fast. It has been a long wait. Although they are small and look young, some of these saplings may be tens or hundreds of years old. They grew slowly in the shade, died back to the roots every few years, then resprouted to creep up again, biding their time until the gap opened and released them.