The Forest Unseen: A Year's Watch in Nature (24 page)

Microbes have more direct methods of competition than mere speed of action. It is no accident that most animals are sickened by a meal of rotting flesh—this sickness is partly caused by poisons that the microbes have secreted to defend their food. “Food poisoning” is impalement on the fence that microbes have erected around their turf. Our tastes have been bent to the microbes’ evolutionary wills; we shun rotten food to avoid defensive secretions. Turkey vultures have not been so easily dissuaded. Their guts burn away microbes in battery acid and potent digestive juices. Beyond the gut, vultures have a second line of defense. Unusually large numbers of white blood cells rove their blood, seeking foreign bacteria and other invaders to engulf and destroy. This swarm of defensive cells is kept provisioned by a particularly large spleen.

Turkey vultures’ strong constitutions allow them to feed where others would gag or sicken. Paradoxically then, the microbes’ toxic barrage benefits vultures to a certain extent by deterring other animals. The line between competition and cooperation is, once again, not so easy to draw.

The vultures’ digestive prowess affects the broader forest community. Because vulture digestive tracts are powerful destroyers of bacteria, vultures take their role as purifiers beyond just tidying corpses. Anthrax bacteria and cholera viruses are killed by passage through a vulture. Mammal and insect guts have no such effect. Vultures are therefore unmatched in their ability to cleanse the land of disease.
Cathartes
is truly well named.

Fortunately for those of us who are not fans of anthrax or cholera, turkey vulture populations are stable across most of their North American range. In the Northeast, vulture numbers have even grown, perhaps as a result of increasing densities of deer, all of which must eventually die and be disposed of. There are two exceptions to this
good news. Parts of the country that have become dominated by soybeans or other row crops have seen vulture populations decline. Agricultural monocultures support little animal life and have little need for undertakers. Another, more subtle threat lies in deer and rabbit hunters’ abandoned or lost kills. Lead ammunition shatters into a fine spray of heavy metal, contaminating shot meat. This is bad for hunters and their families but worse for vultures, which often eat more hunted game than even the most avid shooter. Many turkey vultures are therefore slightly sickened with lead, but the overall population is not in danger from this heavy metal, probably because most vultures have diverse diets that include plenty of nonhunted carrion. In contrast, California condors eat proportionally more lead-peppered bodies than do their cousins the turkey vultures. The few wild-living condors are kept alive by being periodically captured and purged of lead by veterinarians. North American hunting culture necessitates a strange inversion, a purification of the purifiers.

It could be worse. In India, the interaction between technology and vultures has created a much bigger crisis. Widespread use of an anti-inflammatory drug in livestock has inadvertently devastated vulture populations. The drug persists in carcasses and is deadly to the once abundant vultures. Indian vultures are now on the brink of extinction, and as a consequence, putrid dead livestock litters the land. Fly and feral dog populations have exploded, with terrible consequences for public health. Anthrax is common in parts of India. India has the highest incidence of human rabies infections in the world, and most of these are caused by dog bites. The loss of vultures and the subsequent boom in feral dog populations is estimated to cause between three and four thousand extra cases of human rabies per year.

The Parsi community in India has felt the absence of vultures in a different way. Their funerary customs call for the dead to be placed in a Tower of Silence. Corpses are arranged in circles in these squat, open-topped towers where in a few hours vultures turn bodies to bones. Now, with no vultures to consume the dead and with religious proscriptions
against burial or fire, the Parsi community is thrown into an extinction-induced philosophical crisis.

India has learned a hard and undeserved lesson about the valuable work of these bald-headed purifiers. The anti-inflammatory drug that caused this woe is now banned in India, but its use continues in some areas, and the vultures have yet to rebound. Regrettably, the same drug is now making inroads in African countries where vultures appear to be just as important and just as vulnerable.

Here in Tennessee, turkey vultures wheeling over the hills are a common sight. So common that it is easy to forget what a gift we have.

M
igrant birds continue to stream over the mandala. Most are traveling south from the boreal forest, a 2.5-million-square-mile expanse of coniferous woodland that stretches from Alaska, through Canada, to Maine. This forest rivals the Amazonian rain forest in size, and it is the breeding ground for billions of songbirds. As the migrants move across the mandala, they carry the resident birds with them in agitated flocks. I watch from a rock ten meters upslope, looking down on surging groups of warblers, chickadees, and downy woodpeckers. The forest is full of their
chip, chek
, and
cheep
sounds: an army of tinkers.

The birds have shed the wariness of the breeding season and come close. Some approach almost within reach of my arm and grant me a clear gaze at their vitality. Their plumage is exquisite. Wing and tail feathers are crisp, crowns are smooth, and body feathers shine as they slide over one another. The birds’ late summer molt is complete and every feather is perfect.

For the hooded warblers in the mandala’s flock, freshly grown feathers must last a full year. The wear of vegetation, grit, and wind will grind the feathers down, and by midsummer feathers will be ragged-edged and slim. Hooded warblers turn this aging process to their advantage, however. The birds abrade themselves into their breeding costume. Their crowns and throats are muted yellow now, but as the outer edges of these feathers wear away, the black of the breeding
plumage is revealed below. This is a thrifty strategy; most other bird species acquire their breeding colors by growing new feathers, each one of which is made from costly protein.

The chickadees, woodpeckers, and hooded warblers grew their fresh fall feathers here, in and around the mandala, following their summer breeding. But most birds in these flocks molted much farther north, in the spruce thickets of Canada. The names of these species, magnolia warbler and Tennessee warbler, belie their ecology. Both were first described and named from migrant “specimens” in the southern states, and this historical peculiarity is fossilized in their names. The magnolia warbler was shot as it fed in a Mississippi magnolia tree; the Tennessee warbler met its fate along the banks of the Cumberland River in Tennessee. Other boreal breeders bear the same historical baggage. Cape May warblers, Nashville warblers, and Connecticut warblers are all birds of the great northern forests. Thus the conventions of zoological nomenclature hide a great truth about the bird life of this continent. The boreal forest is the nursery of North America’s avian aristocracy, the warblers, the majority of which nest exclusively or mostly in the north. The mandala is washed twice annually by a tide whose volume and power is born in the land of the wolverine and the lynx.

A distinctly southern sound punctuates the tintinnabulation of boreal birds. A yellow-billed cuckoo clucks from the canopy then bursts into a cascade of hollow
kuk
s, drumming out its song. I see the bird high above the mandala, jumping from branch to branch like a monkey. It barely opens its wings as it leaps and cranes its scythelike beak into leaf clusters. It grabs a katydid and gulps the fat insect down before lurching back into the high hidden canopy.

Cuckoos are abundant in the forest around the mandala, but their shyness and their penchant for tall trees mean that they are seldom seen. This bird, like other cuckoos before it, startles me with its strangeness. The cuckoo moves like a primate, sounds like a drummed hollow log, and eats insects that other birds cannot or will not. Its huge beak
allows it to swallow big katydids and even small snakes. Caterpillars’ defensive hairs deter other birds but not cuckoos. Smooth or hairy, everything goes down the gullet, sometimes with a brisk beaking to snap off hairs, but more often caterpillars are swallowed whole, hairs and all. Cuckoo stomachs are apparently densely matted with caterpillar spines whose barbs lodge in the intestinal wall.

Cuckoos make it their business to break other rules of bird behavior. They don’t set up predictable territories but wander nomadically on their breeding grounds looking for clusters of food, then quickly set up camp and breed. The chicks grow rapidly and grow feathers that literally pop open, fully formed. The adults’ molt is a casual affair. Instead of shedding and growing feathers in an organized sequence and at a regular time like other birds, cuckoos molt feathers haphazardly, one by one, and spread the molt over their summer and winter grounds. Perhaps psychoactive caterpillar toxins have loosed their allegiance to the status quo or, more likely, their molt strategy is like their breeding style, designed to take advantage of local bursts of richness, then to coast through the lean times. Even their migratory behavior is loose. Ornithologists in South America have captured very young birds, strongly suggesting that some of the “migratory” cuckoos linger and breed on their wintering grounds.

Of all the birds in the mandala today, the cuckoo travels the farthest. The Amazonian forests east of the Andes are its winter home. Most warblers travel slightly less far, to southern Mexico, Central America, and the Caribbean. So the mandala connects, at this moment, nearly the entire New World. Memories of tapirs and toucans brush past thoughts of the tundra’s edge; minerals from Ecuador and Haiti fly with sugars from Manitoba and Quebec.

Tonight the warblers will link the mandala outward, beyond the earth’s bounds, bringing awareness of the stars into the forest’s matter. Having rested and fed all day, the migrants will wing southward in the cool and safety of dark. These flying birds will scan the skies, find Polaris, the North Star, and use its position to head south. The birds
gained this astronomical knowledge as youngsters, sitting in the nest, peering into the night and searching for the star that did not swing across the sky. They carry this memory in the wet tangle of their brains, then gaze up in the autumn and steer by the constellations.

Remarkable as it is, knowledge of the stars is a fallible method of orientation. Stars are obscured on cloudy nights, and some first-year birds may grow up in dense forests or overcast regions. Migrant birds therefore have several extra navigational skills. They watch the sunrise and sunset, they learn to follow north-south mountain ranges, and they can detect the invisible lines of the earth’s magnetic field.

Migrant birds throw open their senses to the cosmos, integrating sun, stars, and earth as their great tide surges south.

I
sit very still. Time seeps by. A chipmunk walks across the opposite edge of the mandala, barely a meter away. The animal pauses, rummages in the litter with its paws and nose, then disappears into a jumble of rocks. This is a rare encounter. Unlike their suburban or campsite cousins, chipmunks on this mountainside are jumpy creatures. They approach me only after I have sat immobile for a long time. Encouraged by the fruit of my stillness, I settle down and melt into the rock.

Easy breeze. Distant birdsong. The forest’s waters are calm. An hour passes.

Then a sharp, hoarse exhalation of air, just a foot or two behind me. I keep still. The deer blasts out another alarm, then a double blast. A flash of white hits my eye and the animal bounds away, snorting as it goes. The deer’s alarm belly-flops into the smooth, quiet air, smacking sharp energy through the mandala.

The snorts immediately set three squirrels chattering and whining. Eight chipmunks join in, shooting off rapid
chips
. The wave moves out from the mandala. A wood thrush downslope starts calling,
whippa-whippo-whop
, its head feathers hackled up as it hurls out the call. Distant chipmunks pick up the staccato chorus, carrying it to the edge of earshot.

The deer’s alarm at coming so suddenly on an immobile human has reached out hundreds of meters. The agitation, particularly that of the chipmunks, takes more than an hour to recede.

The mandala’s birds and mammals live embedded in an acoustic network, each individual connected to others through sound. The forest’s news ripples through this network, carrying the latest information about the location and activities of troublemakers. It takes some effort for us urbanized humans to become aware of these traveling signals. We are accustomed to ignoring “background noise,” instead taking our cues from the interior noise of our minds. Most of my time sitting or walking in the woods is spent riding waves inside my head, thinking of past or future. I suspect that this is a common experience. Only a repeated act of the will can bring us back to the present, back to our senses.

When we arrive in the acoustic now, we discover that the forest’s newsroom is focused on— surprise!—us. We’re large, noisy, and fast. And many animals have seen us in our more predatory modes. Those that haven’t had personal experience of our guns, traps, and saws quickly learn from their more experienced peers: it is in an animal’s interest to pay attention to what alarms others. We are like the hawks, owls, and foxes that seldom get to observe the forest network without triggering noisy news bulletins. Sitting low, staying still, and biding one’s time is the only way to slip in. Then we experience the alternating calm and clatter of the news wires. Hikers, for example, are preceded by bow waves that arrive minutes before their chatter and laughter. More minor disturbances, such as a branch falling or the overflight of a crow, send quieter and more short-lived pulses through the network. The deer’s alarm at stumbling upon me was, on the other hand, a surge, a bold headline.