The Kingdom of Rarities (13 page)

If monkeys, macaws, and jaguars are so ecologically valuable, how can we make them worth more alive than dead to the local people—those who live in or at the edge of the rain forest and rely on its bounty? George has been working closely with Adrian
Forsyth and their Peruvian colleagues to design a network of parks and conservation areas to keep this Amazonian region as intact as possible. If jaguars need a lot of space and the largest reserves are still too small to hold what biologists consider a genetically healthy population, linking reserves through corridors jaguars use can greatly enhance the conservation effect. Maintaining connectivity between reserves increases movement and ultimately gene flow. In the Amazon, such landscape-scale conservation with respect to large predators and other roamers is still possible with the proper incentives. One promising program, called the wildlife premium mechanism, would pay poor Peruvians who collect Brazil nuts for a living to become stewards of the forests and protectors of jaguars and pumas. By agreeing to maintain the dispersal corridors for jaguars among Manú National Park, Tambopata National Reserve, and nearby Alto Purús National Park, the communities could help secure a landscape of several million hectares and receive financial benefits. The plan, which is still in its infancy, may hold a key to a future with jaguars and pumas and the best inoculation to a cascading decline of Amazonian diversity beyond its treasured parks and reserves, where jaguars still venture out onto the beach.

A
N UNBROKEN VISTA OF STUNTED JACK PINE
trees dotted the northern Michigan landscape. Maybe our guide had mistakenly led us into a Christmas tree farm. The only sizable trees in view were a few skeletons—blackened snags that remained after a blaze a few years back. This unremarkable setting seemed like the last place one would search for rarity.

“Keep watching the snags,” advised our escort. “The males prefer them as singing perches.” A few small songbirds flitted about the pine grove and our group of birders snapped to attention. False alarm. The distant silhouettes became sparrows and juncos—perfectly nice birds but not the ones we were here to see.

Then, almost simultaneously, three male Kirtland's warblers rose out of the dense underbrush and perched, one per snag, to advertise their individual glory. A nearby singer threw back his head and uttered the anthem of the jack pine, a loud and lively “
Flip lip lip-lip-tip-tipCHIDIP!
” Within seconds, a fortune's worth of spotting scopes, massive telephoto lenses, and binoculars were trained on the handsome male. The fifteen-centimeter-long bird sported a bright yellow breast streaked in black; a dark mask over his face, highlighted by white eye rings; and a slate-blue back—a perfect marriage of classic understated plumage and a splash of color. We could hear more singing males in the distance, all staking out their territories before the females arrived. “This population is rebounding rapidly,” the guide was telling us, “and the current estimates for this year are about 1,770 singing males in the wild.”

Gleeful smiles signified our good fortune on that May day in 2010. For almost everyone in the group, this was a first-ever sighting, a “life bird.” Some had traveled across oceans to record the event. Secondary for them was the news that this stunning bird was staging a comeback. But that was the purpose of my visit—to understand better how rare species with particular habitat preferences manage to persist and even recover when perched on the edge of extinction.

If a shrine to rarity exists, the jack pine woods near Grayling, Michigan, is nature's Lourdes. Each spring, thousands of birders journey to this small town in the state's Lower Peninsula, about five hours' drive north from Detroit, to encounter the rarest breeding songbird in North America. Territorial males return in early May, having winged in from their wintering grounds in the Bahamas. The females linger on the islands a bit longer, fattening on fruit and insects, before beginning the perilous 2,500-kilometer migration back to their only significant breeding site. From the exotic Caribbean islands to the stunted jack pine woods on the outskirts of Grayling, such is the seasonal arc of a most uncommon species.

The day after our birding excursion, a group of young marchers fidgeted under the bright morning sun, eager to start the celebration. It was the third Saturday in May, time for the annual Kirtland's Warbler Wildlife Festival. A roving reporter asked a young girl in line, “Do you know why the Kirtland's warbler is so rare?” In
response, she could have pinpointed part of the answer by quoting a verse from Dr. Seuss, the one they teach in science class about the Nutches, “who live in small caves, known as Niches.” Unfortunately for the Nutches, there simply weren't enough niches to go around.

The singing male Kirtland's we witnessed in the pine grove had to compete for his niche, limited breeding space. Kirtland's warblers have historically nested only in stands of small-stature jack pine maintained by natural fires. This extreme preference about where they will or will not make their nest gives them, and a large number of other species, a special place in the Kingdom of Rarities. There are species that have limited ranges, such as the golden-fronted bowerbirds, and those that live at low densities, such as the jaguars. The Kirtland's warbler, singing boldly from its perch in a fire-prone stand of young jack pines, meets those conditions and raises the ante. Beyond having a narrow range and low numbers, it represents another dimension of rarity: extreme habitat specialization. By this I mean a species that breeds only in a particular habitat, feeds only there, or requires some other feature there to thrive. That could be a cool, dry, or wet sanctuary to maintain an ideal body temperature or the proximity of a river or stream from which to drink several times per day.

In the grand scheme of nature, many species are quite particular in their habitats. Arctic lemmings and other species that reside in the tundra could not live anywhere else, for example. Parrot fish are literally a fish out of water beyond their preferred tropical coral reefs. Even the ubiquitous prairie dog could not survive anywhere except a grassland. All these habitat specialists are, fortunately, adapted to living in an ecosystem that is widespread in the world, at least today. But others, including the Kirtland's warbler, are specialists on highly restricted habitats. I came to see how such extreme specialists among species persist and recover, but I also wanted to explore in what circumstances such a narrow habitat bandwidth becomes a liability. All of the patterns now present could be in flux: as climate change scientists are telling us, the distributions of preferred
habitats may change dramatically over the coming century. Those species that are specialists on widespread habitats and are common today may find a vastly shrinking range tomorrow. The narrowrange Kirtland's warbler may thus be a bellwether species to help us better understand how species cope in a more restricted area.

Extreme habitat fidelity: is it a cause of rarity itself, or a condition of it? Actually, it can be either or both, depending on the circumstances. The linked questions have been central to the debate on what lies behind patterns of rarity and abundance in nature. Our understanding of extreme habitat fidelity as a condition of rarity owes much to the work of the population biologist Deborah Rabinowitz. In 1981, she developed a way to think about rarity that remains influential today: to a species' range size and its population density, she added a third quality—its loyalty to a particular habitat—as a condition of rarity. One thing is clear: if highly selective breeding habitat alone explained a major piece of the rarity puzzle, Dr. Seuss's classic could replace bookshelves of ecology texts. But there is much more to the story. For example, if today you must go to Grayling to see Kirtland's warblers with ease and in appreciable numbers on their breeding grounds, time travel back before the last ice age might well have offered more viewing options. Some scientists believe that this species was probably more common prior to the last glacial period, when the jack pine forests were more widespread. Grayling sits on the edge of the bird's historical range. Some biologists predict that under several climate change scenarios projected for northern Michigan, the future for this species could be greatly curtailed, but that still remains conjecture.

At its low point, in 1971, the global population of Kirtland's warblers dropped to around 400 birds. At that time, only about eighteen square kilometers of suitable immature jack pine habitat remained, and that was in Michigan. The warbler joined the first US endangered species list in March 1967 as one of seventy-eight native animal and plant species then under threat of extinction, and it remains America's most imperiled breeding songbird.

How frequent is the condition of extreme habitat specialization in nature, and how might interventions on behalf of the Kirtland's warbler inform efforts to save species with similar distributions? Viewed through our magnifying lens of rarity, maybe the seemingly boring Christmas tree farm held a lot more interest after all.

Standing in the midst of the jack pine forest on that May day, I still couldn't get over the sameness of it all. Having lived and worked in species-rich tropical rain forests, I have a natural aversion to monocultures. If the view from the spotting scope brought rarity into sharp focus, the naked eye swept a panorama that seemed like a biological wasteland. Why did the Kirtland's insist on nesting
here
? If I pretended to be a northern Seuss, I might ponder:

What's this business about jack pines, do tell!
Why so picky where you pick to dwell?
And why nest on the ground
where skunks abound,
and heavy rains cause your chicks to drown?
Mrs. Kirtland, why not raise them in town?

And what was the fate of those birds that failed to find a home in a young jack pine grove? Did their offspring survive at all? Skunks and other midsized nest predators do eat Kirtland's chicks. But did chicks survive better when such top predators as wolves, bears, and pumas kept the population of skunks, raccoons, and their eggsnatching ilk in check? My field notebook held more questions than answers.

The singing male above me kept at it. In the heart of his territory were huckleberry bushes, aromatic sumacs, and sweet ferns (a shrub in the bayberry family), but the tree layer was virtually a pure stand of young jack pines. The pines were hunched and distorted, partly because they were growing on Grayling sands, an acidic, porous, nutrient-poor soil. The trees were about eight years old, perfect for this warbler, which builds its cuplike nest on the ground under
the spreading pine boughs. Saplings younger than five years of age offer poor cover, while the upper branches of jack pines older than twenty years block the sun from reaching the ground. At that age, too, the lower branches of older trees drop off, and overhanging grasses become shaded out. This exposes the ground nests to predators, which besides skunks include snakes, thirteen-lined ground squirrels, and blue jays. Kirtland's are also particular about substrate, the surface on which an organism grows or is attached. Rainwater percolates quickly through Grayling sands, so shielded nests and the surface soil layer stay dry even after summer cloudbursts. Nestlings left sitting in puddles or on waterlogged soils suffer heavy mortality.

Male Kirtland's warbler (
Dendroica kirtlandii
) on a jack pine branch

The bold male in the snag spent the next fifteen minutes throwing back his head, bobbing his tail, and filling the air of the jack pine grove with his lively song. Such lovely plumage in the bright sunshine brought cheer to everyone. Nearby, Nashville warblers began singing. Kirtland's and Nashville warblers, along with the other fifty-four species of New World warblers that breed in the United States and Canada, are part of nature's balancing mechanism, as these dedicated caterpillar eaters, when gathered in numbers, clean the forests of larval insects harmful to trees. They are also among the most stunning of pest removers. Natural selection, using delicate brushstrokes and a colorful palette of yellow, green, chestnut, orange, red, blue, and black, has fashioned an exquisite array of tiny creatures—only one warbler species is larger than a sparrow.

Unlike most other members of the warbler family, the Kirtland's was discovered relatively late in the history of North American ornithology. It was first described by taxonomists in 1851, when a male was collected on the outskirts of Cleveland, Ohio. The songbird was named in honor of Jared P. Kirtland, an Ohio physician, teacher, horticulturist, and naturalist who assembled the first lists of vertebrates for the Buckeye State. Ironically, by the time the Kirtland's warbler was named, it was already experiencing a range collapse, and by the time of Kirtland's death, in 1877, it may have disappeared entirely from Ohio.

The bird's wintering sites first became known in 1879, when a specimen was collected on Andros Island in the Bahamas. Since then, wintering Kirtland's warblers have been found on other islands in the Bahamian archipelago, on the Turks and Caicos Islands, and on Hispaniola. Yet only in 1902 did biologist Norman Wood find the first nest, in Michigan's Oscoda County. Like many of the more than 200 species of migratory songbirds that nest in the United States and Canada, the Kirtland's warbler actually spends eight months of the year on its wintering grounds.