Unsolved Mysteries

When It Became a New Species, a Tiny Songbird Was Thrust into the Limelight

06-01-1998 // Harry Thurston

The fog is so thick on Vermont´s Mount Mansfield that the communications tower crowning this 4,400-foot peak disappears in the gray ether. All around is a wind-stunted forest of gnarled spruce and fir. The rising sun glows as weakly as a 15-watt light bulb. Riffling through this eerie atmosphere is the buzzing song of a rare, olive-hued bird, Bicknell´s thrush.

Christopher Rimmer, research director of the Vermont Institute of Natural Science (VINS), and his colleagues are here on this bleak June day to gauge how sensitive Bicknell´s thrush is to human disturbance. This subject became hot almost overnight in 1995 in the Northeast when the American Ornithologists´ Union (AOU), the official arbiter of avian names, declared Bicknell´s thrush a distinct species. Suddenly, this obscure migratory songbird was thrust into the limelight as a conservation priority.

"People are now saying, ´What´s this going to do to Bicknell´s thrush, or what´s the status of Bicknell´s thrush in the project area?´" says Rimmer, whose team is conducting studies of the species. "We don´t have those answers and everyone wants them, right now. But they´re not going to come quickly."

Answers regarding Bicknell´s thrush have never come quickly. It was first observed in 1881 by ornithologist E.P. Bicknell, who noted a "hitherto unknown thrush" on Slide Mountain in New York´s Catskill Mountains. This songster was quickly consigned to taxonomic obscurity, however, when it was classified as a subspecies of the more widespread gray-cheeked thrush, which breeds in boreal forests from Alaska to Newfoundland.

Interest in the status of Bicknell´s thrush was revived in the 1930s, when University of Michigan doctoral student George J. Wallace demonstrated differences in size and color between Bicknell´s and the gray-cheeked thrush. But it was not until the 1990s that Henri Ouellet, researcher emeritus at the Canadian Museum of Nature, proved that Bicknell´s thrush truly was a bird of a different feather. Ouellet found color differences in the plumage of the Bicknell´s and gray-cheeked thrushes. More importantly, he showed that the breeding and wintering ranges of the two birds did not overlap; the songs, too, were decidedly different; and there was no evidence of interbreeding between the two. DNA analysis indicated that the two species had diverged genetically one million years before.

This evidence convinced the AOU to grant distinct species status to Bicknell´s thrush in 1995, when several other species were also officially recognized. (The AOU publishes bird-species updates about every two years, most recently in 1997.) "It suddenly brought attention to this bird and has generated additional research," says Richard Banks, chair of the AOU´s checklist committee and an ornithologist with the U.S. Geological Survey. "People are more likely to pay attention to a full species than a subspecies."

Conducting research is difficult, not only because Bicknell´s thrush is rare but because it prefers to nest in thick, remote forests. "You put a band on a bird and you may never see it again--even though it´s there," says Rimmer. Last summer, however, he temporarily held a female that he had banded in 1992, after she returned to the same nesting site.

Building on Wallace´s pioneering work in the 1930s, VINS researchers have started to sketch a life history for this elusive bird. The thrushes arrive in New England in late May and begin to nest by the first or second week in June. They build compact nests insulated with moss and concealed against the trunks of fir trees, six to ten feet above the ground. Females incubate the eggs, which usually hatch by the Fourth of July. Eleven to 12 days later, the chicks fledge, if they are lucky. Many eggs and chicks fall prey to predators; only 17 of 33 nests Rimmer´s team observed fledged young.

With the thrush´s nest-success rate already low, conservationists wonder if new towers and ski trails planned for Mount Mansfield and other northeastern peaks will imperil the species. Even mountain biking and hiking may disturb the birds and diminish their chances of reproducing. "Each project individually may not have a really big impact on any one site, or on the population as a whole," Rimmer says. "It´s just a general whittling away of the habitat, which is a limited one to begin with. And we don´t know what effect fragmenting has."

Since Bicknell´s is an island species at both ends of its range, it is particularly vulnerable. In summer, it breeds on "sky islands" atop the Appalachians in New England and in adjacent Canadian provinces. In winter, it returns to a few Caribbean islands, including Hispaniola (divided between the Dominican Republic and Haiti), Puerto Rico, Jamaica and perhaps Cuba. And these wintering grounds are disappearing. Only 13 percent of native forests still stand in the Dominican Republic and less than 5 percent remain in Haiti. "Some sites where we found the thrushes last winter were being cut down as fast as we could count the birds," says Rimmer.

With the thrush´s habitat shrinking in the north and south, Partners in Flight (PIF), a group of public and private organizations concerned with bird conservation in the Americas, has designated the bird as the highest conservation priority among neotropical migrants in the Northeast. "We need to find out more about them and we need to know how their populations are varying," says Ken Rosenberg, PIF´s regional coordinator for the Northeast.

Currently, no one even knows how many Bicknell´s thrushes survive. The total population may be as few as 10,000 or as many as 40,000 pairs. VINS research may determine if the thrush is rare enough to be a candidate for listing under the Endangered Species Act.

This fog-drenched morning on Mount Mansfield, Rimmer continues his own search, struggling through the lattice of fir and spruce. He returns to the road to check his bearings and to take a compass reading, before once again plunging into the thick undergrowth. He is searching for a nest he discovered the week before.

"Oh, that´s too bad," he says, as he finally spots the nest. The eggs lie cold and wet, abandoned by the female Bicknell´s thrush. "They are sensitive."

Canadian Harry Thurston interviewed scientists in Vermont for this article.

Join today and get a 1 year subscription to National Wildlife magazine
     Flickr Icon           Find NWF on Facebook.           Follow NWF on Twitter.           YouTube Icon    
Connecting...
Certify your yard today!