Amid a sea of scrubby jack pine, several scientists gather as a male Kirtland’s Warbler—one of several within earshot—sits on the tip of a tree branch and fills the sky with his buoyant, clear song. The large warbler, with dandelion-yellow breast and slate-blue head and back, stands out as clearly as a traffic sign. It seems somehow incongruent that one of the continent’s rarest migratory birds would sing so boldly in plain view, only 30 feet away.
“Oh, we can get way closer,” marvels Nathan Cooper, a researcher at the Smithsonian Migratory Bird Center. “They just sing. They seem to sing all the time.”
The spirited male makes no effort to hide or evade. “They just don’t know any better,” adds Peter Marra, the head of the Smithsonian center and an expert in songbird migration and population dynamics.
The task of finding a singing Kirtland’s Warbler on its nesting ground would have been vastly harder three decades ago. At that time, the entire known population numbered only 167 breeding pairs scattered across the jack pine barrens of the northern portion of Michigan’s Lower Peninsula. But in the years since, the boisterous warbler has multiplied to more than 2,000 pairs and has spread—albeit tentatively—to Michigan’s Upper Peninsula, northern Wisconsin, and across the border to Ontario.
“I think it’s a tremendous success story,” says Marra. “Are we out of hot water? No, I would say we’re not out of hot water, especially given some of the threats that we’re seeing during the nonbreeding season, like climate change and sea level rise,” which could affect the warbler’s wintering habitat in the low-lying Caribbean.
“So it’s important that we continue to do this kind of research to really understand as much as we can about the species and to get the population size as high as we possibly can so it’s safe, not just for the next 50 years but maybe 200, 300 years.”
The success and upward trajectory of the warbler’s numbers has prompted the U.S. Fish and Wildlife Service to consider downlisting the Kirtland’s from “endangered” to “threatened” on the federal endangered species list, or removing it altogether.
But hurdles remain. And even if the path is cleared for removing endangered species protections entirely, the Kirtland’s Warbler will remain utterly dependent on continued human maintenance of its breeding habitat.
“The Kirtland’s Warbler is one of those ‘conservation-reliant’ species,” says Christie Deloria, a Fish and Wildlife Service biologist and member of the Kirtland’s recovery team who has worked with the species for more than 20 years.
“This is a species that’s going to need our attention into the future,” she says. “There are many other listed species that are in the same boat. But what’s unique maybe with Kirtland’s is that we’ve had huge success at recovering the species. And we are perhaps one of the first groups to be figuring out how do we take a conservation-reliant species off of the endangered species list.”
Habitat Loss and Cowbird Eggs
The Kirtland’s is jumbo by warbler standards—up to 15 grams, about twice the size of an American Redstart. It was named for an Ohio doctor and naturalist (Jared Potter Kirtland), but sometimes it is called the “jack pine warbler,” because it habitually nests on the ground beneath young jack pines. And it is there that the long story of its decline and near-extinction begins.
Each spring Kirtland’s Warblers fly north from the Bahamas to Michigan, where they pair up and nest almost exclusively in large tracts of jack pine between 1 to 4 meters (3 to 12 feet) tall. Marra says it’s not clear why Kirtland’s Warblers will nest only among young trees. But it is clear that once the stand reaches about 15 years old, the birds disappear.
It’s also not clear if the warbler’s reliance on young jack pine evolved since the last glacial retreat about 10,000 years ago, or if the bird has followed jack pine forests at glacial margins through several ice ages. But the warbler and jack pine are inextricably linked now, and the only natural way to create sweeping stands of scraggly young jack pine is through forest fire.
In the past, fires were ignited by lightning and also by indigenous peoples to create openings for game and forest crops, such as blueberries. The Kirtland’s was probably never common, even in its Michigan range. Its numbers may have peaked in the late 1800s, when wildfires scorched the land in the wake of pine logging. By the 1920s, settlers got serious about stomping out wildfire. The amount of land in warbler territory that burned each year fell from an average of 14,000 acres (based on fire records) to only about 1,000. Jack pine grew too old and tall to interest warblers.
Vanishing habitat wasn’t the only problem. By the early 1900s, Brown-headed Cowbirds were becoming common, taking advantage of the openings created by logging and agriculture. Cowbirds lay their eggs in the nests of other birds, and the cowbird young hatch early and monopolize food. The first instance of cowbird parasitism of a Kirtland’s Warbler’s nest was reported in 1908. The warbler seemed unable to identify the intruders or remove their eggs or even defend its own young. Parasitism rates ranged from 48 to 86 percent of Kirtland’s Warbler nests, according to Fish and Wildlife researchers William Shake and James Mattsson. Kirtland’s nests on average produced fewer than a single warbler fledgling.
As fire-generated habitat disappeared and cowbirds invaded, the number of warblers plummeted. A 1951 census counted only 432 singing males in 28 townships. By 1971, the number had fallen to 201 in just 16 townships. In 1974 and again in 1987, the population hit its nadir at 167 singing males.
A High-Maintenance Project Pays Off
As early as 1957, state and federal wildlife managers began creating more jack pine stands to benefit warblers. In 1973, the Kirtland’s Warbler became one of the first species protected under the Endangered Species Act. A recovery team of federal and state biologists aimed to “reestablish a self-sustaining Kirtland’s Warbler population throughout its known range at a minimum level of 1,000 pairs.”
Foresters created habitat through clearcutting and then prescribed burning. But the thousands of homes, cabins, and trailers tucked into the woods along dirt roads and country highways made fire a risky bet. The Mack Lake fire in 1980 forced a major rethinking in strategy.
On the morning of May 5, a district ranger on the Huron National Forest gave the okay to light a prescribed burn in southern Oscoda County. As described in a 1981 article in American Forests magazine, relative humidity was low and the temperature, already 74, was forecast to rise during the day. Several spot fires broke out along the perimeter of the burn. Shortly after noon, fire leapt into the crowns of jack pine along Highway 33. By 12:20 p.m. the call went out that the Forest Service needed help. The fire was out of control.
Seven hours later, according to American Forests, nearly 25,000 acres were charred. Forty-four homes and cabins were damaged or destroyed. A Forest Service fire technician was dead. Local resident Joe Walker remarked as he raked through the remains of his cabin, “I hope that warbler enjoys his nest. My nest is burned.”
Mack Lake made the use of fire fraught, if not impossible. Instead, land managers developed a method of clearcutting and mechanical planting, incorporating half-acre openings among the rows of jack pine. Currently, about 4,000 acres are harvested and replanted each year, about half on state land and half on the Huron-Manistee and Hiawatha National Forests. The timber management is purely in the name of habitat—young jack pines have very little commercial value.
Creating habitat was one thing, but keeping cowbirds away was another. In 1972, the Fish and Wildlife Service began trapping cowbirds and quickly reduced parasitism to less than 10 percent of Kirtland’s Warbler nests. The rate has continued to drop as the agency has spent roughly $100,000 a year to capture and euthanize an average of nearly 4,000 cowbirds each season in Michigan.
That two-pronged approach did the trick. By the late 1980s, Kirtland’s Warblers began to increase in number. Annual production of young has more than tripled to 3.52 young fledged per nest attempt. The species surpassed its recovery goal of 1,000 breeding pairs in 2001 and has doubled since then.
Despite this success, the Kirtland’s Warbler will continue to be a high-maintenance project—probably for as long as warblers and humans share the continent. Unlike, say, the Bald Eagle, a bird that could take care of its own recovery after humans stopped using DDT, the Kirtland’s Warbler will depend on the high-cost creation of its nesting habitat and ongoing cowbird control.
Money to maintain habitat and trap cowbirds is available through the warbler’s designation as a federally endangered species. But “when we take it off the endangered species list, that money goes away,” says Deloria, whose first job with the Fish and Wildlife Service was running the cowbird trapping program.
That’s where Marra’s research with the Smithsonian Migratory Bird Center comes in. “This study that they are doing is really important,” says Deloria. “If we could operate fewer traps on the landscape, and the program costs less to operate, maybe we don’t have to raise as much money.”
Linking Summer and Winter Habitats, and the Places In Between
Marra has stalked the Kirtland’s nesting grounds for several years now. In a study published in 2009, Marra and PhD student Sarah Rockwell examined isotope signatures found in the blood, feathers, and nails of Kirtland’s Warblers to determine the conditions of their wintering habitat in the Bahamas. They recorded how early the birds arrived on their nesting grounds and how many fledglings they produced. Wetter years in the Bahamas meant more fruit and insects for warblers, earlier migration, and better reproductive success 1,400 miles north in Michigan.
“To some degree, Bahamian rainfall drives reproductive success [in Michigan],” says Marra. That knowledge may become ever more critical as sea level affects the low-lying Bahamas and a changing climate affects precipitation patterns.