Science Sleuths

A little gray bird offers some big lessons

• Peter Friederici
• Oct 01, 2003

COSTA RICA IS NO BIGGER than West Virginia, yet this small Central American country boasts as many bird species as all of America north of the Mexican border. Birders flock to the country's cloud forests and lowland jungles to see such coveted species as toucans, motmots, hummingbirds, resplendent quetzals and scarlet macaws. Compared to these brilliantly colored and exotic birds, the willow flycatcher seems hardly worth notice.

A little gray bird only six inches tall, it summers in the United States and Canada, and spends the remainder of the year in Central and South America. In the past, chances are, no one traveled from the United States to Costa Rica specifically to see willow flycatchers—no one, that is, until Tom Koronkiewicz, a former U.S. Geological Survey (USGS) biologist, began studying the species outside its summer range. Thanks to his research, the intimate links that connect the avian life of Central America with that of the United States and Canada have become clearer than ever before.

Koronkiewicz began his work in 1997, two years after the U.S. Fish and Wildlife Service had listed the southwestern willow flycatcher—one of four subspecies of the flycatcher—as endangered. The various subspecies look identical in the field, but the southwestern variety had become rare because many of the riparian areas from Southern California to West Texas in which it nests had become degraded. Experts believed the bird's numbers had been reduced to only about 500 breeding pairs.

The endangered listing helped produce a flow of funding into research on the subspecies, including banding and intensive monitoring of some of the breeding populations in the Southwest. But the money was spent almost entirely north of the border. "Almost nothing was known about the bird on the wintering grounds," says Mark Sogge, a USGS biologist in Flagstaff, Arizona, who oversees the agency's willow flycatcher research. "That's a real problem when you're supposed to manage something that spends more time on the wintering grounds than it does here on the breeding grounds."

With the help of museum specimens collected long ago in Central America, Koronkiewicz was able to identify a few general areas where willow flycatchers might congregate in winter. From studies north of the border he knew that willow flycatchers nest in thick riparian vegetation, especially where tall trees overlook wet areas. So he went looking for wintering sites that looked similar.

In Guanacaste, in northwest Costa Rica, he found dozens of such sites. Guanacaste is a low-lying, largely agricultural province located on the Pacific coast that is rarely visited by ecotourists. Koronkiewicz found broad pastures and sugar-cane fields surrounding the area's isolated wetlands, but it didn't take long for him to hear the distinctive fitz-bew call of willow flycatchers. He began in-depth studies of the birds at two sites of several dozen acres apiece.

The wetlands were tough places to work. At the end of the wet season, in November and December, they were flooded and thick with mosquitoes. They dried out in the months that followed, but then Koronkiewicz and his field assistants were forced to crawl through dense, thorny shrubs to reach flycatcher territories. "If it's buggy and miserable and impenetrable, if it shreds your clothes," he says, "then you know you're in willow flycatcher habitat."

By temporarily trapping individual birds and placing colored bands on their legs, the researchers were able to identify specific territories—and they found that those territories were indeed specific. Each willow flycatcher pair defended a territory and aggressively excluded others of their species from it. The birds had extremely regular habits. "I got to know the birds so well that I could predict exactly where they would be, even what perch they would be on at a given time of day," Koronkiewicz says.

During his second winter in Costa Rica, the researcher found that such predictability also extended between years, as the majority of the flycatchers he monitored the previous winter were back on exactly the same territories. Most remarkably, one of the new birds he trapped in January 2000 was a female southwestern willow flycatcher banded in July 1999 by colleagues at Roosevelt Lake in Arizona. "That's a pretty rare event," he says. "The recovery rates for banded birds are below .0001 percent. In terms of odds, this was like winning the lottery and getting struck by lightning on the same day."

Even more remarkable, notes the scientist, is that the same bird has shown up at Roosevelt Lake each summer since and in Costa Rica each winter. By late last year the bird was four and a half years old, had raised five young and had returned to exactly the same wooded wetland patch every autumn.

"In the United States, we tend to assume that these birds just appear each spring," Koronkiewicz says. "This female has stayed on the same exact wintering territory for eight months a year for four years." That, he points out, represents the longest such period recorded to date for a long-distance migratory passerine.

To Koronkiewicz, this extraordinary habitat fidelity underlines the need to preserve both breeding and wintering areas for Neotropical migrant songbirds, as they may be equally picky about both places. In late 2000, he faced an object lesson in that possibility. One of his winter study sites was adjacent to Palo Verde National Park, which the Costa Rican government wanted to put a road through to provide better access to the area. The road would bisect the wetland where the Roosevelt Lake female wintered. Koronkiewicz and Sogge were upset, and so were local villagers who didn't want their rural life-style disrupted.

"The villagers urged us to write letters to government officials explaining that an endangered subspecies winters at the site," Koronkiewicz says. The campaign worked. Not only was the road project scrapped in 2001, but the wetland was also granted full federal legal protection.

"This was real grass-roots conservation work," Koronkiewicz says. "One little bird helped preserve habitat 40 degrees south of where we originally found her." He notes that this helped much more than just one subspecies, since he has found more than 200 bird species in the wetlands. That includes several dozen other species that migrate north for the summer, as well as scarlet macaws, a Costa Rican showpiece species. Ecotourists visiting the Central American country may not be changing their itineraries anytime soon to see the flycatcher, but the little gray bird is making its presence known.

Arizona writer Peter Friederici is a frequent contributor to this magazine.