Here is a question for you. How many readers of this newsletter have actually seen a Bicknell’s Thrush? Raise your hands. I see a few hands waving in the back, but not very many. Likewise, when I searched the archives of the VTBird list for reports of Bicknell’s Thrush, I found many sightings of Wood Thrushes and Hermit Thrushes, and even a few reports of Varied Thrushes and a Swainson’s Thrush or two. But sightings of Bicknell’s Thrush were few and far between.
Right: Drawing by Gabe Willow, VINS intern
Unless you are willing to climb Stratton Mountain in the rain in late Spring or put up a mist net at dawn on the top of Mount Mansfield during breeding season, Bicknell’s Thrush is an elusive quarry. Yet they are out there surviving in the balsam fir understory, a hidden jewel in the montane forest.
Bicknell’s Thrush is named for an amateur ornithologist, Eugene Bicknell, who in 1881 collected (i.e., shot) a small thrush, whose call was distinctively different from that of a nearby Swainson’s Thrush. Bicknell was hiking on Slide Mountain in the Catskills when he acquired this specimen, which he promptly sent to Dr. Robert Ridgway at the American Museum of Natural History for identification. Ridgway believed that this little bird represented a subspecies of the Gray-cheeked Thrush and gave it the name Bicknell’s Thrush.
In the 1930’s Dr. George Wallace, as a graduate student, performed careful morphological studies comparing specimens of Bicknell’s Thrush with those of the Gray-cheeked Thrush. He noted that the Bicknell’s Thrushes were consistently smaller than the Gray-cheeked specimens he examined and that the coloration of their bills differed as well. Nonetheless, there was considerable overlap in these measurements, and distinguishing between these birds in the field was impossible. Accordingly, he too concluded that the Bicknell’s Thrush was a subspecies of the Gray-cheeked Thrush.
Right: Bicknell’s Thrush range map. Dan Busby, Canadian Wildlife Service
And there things stood until 1993 when Dr. Henri Ouellet, a researcher in Canada, published his findings, suggesting that Bicknell’s Thrush and the Gray-cheeked Thrush, though remarkably similar in appearance, do indeed represent different species. Ouellet based his conclusions on his observation that these two birds consistently differed in size and plumage, that their calls and songs were strikingly different, and that their breeding and winter ranges failed to overlap. Subsequently, analysis of DNA from selected birds clinched the argument that these birds represented different species that had diverged from a common ancestor perhaps a million years ago. In 1995 the American Ornithologists’ Union recommended that Bicknell’s Thrush be recognized as a separate species with the scientific name Catharus bicknelli. And so it was.
A major factor that endears Bicknell’s Thrush to Vermonters is its restricted breeding range. These birds nest in the montane forests of Vermont, New Hampshire, Maine, and New York and in the second-growth conifer forests of eastern Canada. That’s it. Even more than the Hermit Thrush, Vermont’s state bird, the Bicknell’s Thrush is Vermont’s thrush. If you are going to add a Bicknell’s Thrush to your life list, Vermont is a good place to go. But not just any place in Vermont. Typically, Bicknell’s Thrushes are found in mountainous terrain, hidden in the understory amid stunted balsam fir and red spruce at elevations generally above 3000 feet. Recently, researchers at the Vermont Institute of Natural Science (VINS) released a new map of Bicknell’s Thrush habitat which can be viewed on their web site. What strikes the casual observer about this map is just how limited Bicknell’s Thrush habitat is. This useful web site also lists the mountains in northern New England and New York where Bicknell’s Thrushes have been found. Target one or more of them and you will have a chance to observe this reclusive creature. But do not be surprised if your search for Bicknell’s Thrush fails despite your best efforts. These are rare birds, listed on the National Audubon Society’s Watchlist and given the highest conservation priority of the neotropical migrants in the Northeast United States by the international organization Partners-in-Flight.
The reproductive habits of the Bicknell’s Thrush are unusual, too. Not to get too personal here, but it appears that the young birds occupying a single nest regularly have different fathers and that a single father may have young in multiple nests. Whew. Typically, the nest, an open cup of twigs and moss constructed in a small tree, contains three or four small bluish-green eggs with light brown speckling. Once hatched, the chicks are fed by multiple males. Given the rugged terrain that the Bicknell’s Thrushes occupy, perhaps this unusual mating pattern (for songbirds anyway) confers a survival advantage.
Come winter the Bicknell’s Thrushes are gone. Bicknell’s Thrush is a neotropical migrant, which migrates from its breeding grounds in the Northeast to its winter home in the Caribbean islands of the Greater Antilles, mainly Haiti, the Dominican Republic, Cuba, Puerto Rico, and Jamaica. In fact, VINS researchers have twice recaptured Vermont-banded birds wintering in Hispaniola. Amazing. Unfortunately, all is not well in Hispaniola. Though Bicknell’s Thrush is protected by law there, government authority is weak in both Haiti and the Dominican Republic. Habitat degradation represents a major threat to this species. The Environment Canada web site (www.ns.ec.gc.ca), from which much of the information for this article was obtained, provides an excellent map showing breeding areas in the U.S. and Canada, migration routes, and wintering areas.
How would you recognize a Bicknell’s Thrush if you saw one? My Sibley’s guide says that Bicknell’s Thrushes are small, sparrow-sized birds with warm brown coloration and a distinctly reddish cast to the tail. Like the Gray-cheeked Thrush and the Hermit Thrush, Bicknell’s Thrush is a spotted thrush, with a heavily-spotted, buff-tinted breast and grayish-white underparts. The extended yellow color at the base of the bill and the vague, grayish eye ring help to differentiate Bicknell’s Thrush from the more common Hermit Thrush. In other words, good luck.
Right: Chris Rimmer. Baoruco National Park, D.R.
More hopeful as a means of identifying Bicknell’s Thrush in the field is its unique song, which is commonly referred to as haunting. Sibley’s guide describes this song as high-pitched, nasal, and wiry with the middle phrase descending and the last phrase rising. Repeat after me, “ch-ch zreee p-zreeew p-p-zreee”. The call is a sharp, descending peeez. The Cornell Laboratory of Ornithology web site has a nice section on bird identification that includes bird songs and calls. Practice up before you take to the field. For the tone-deaf like me, the only hope for identifying a Bicknell’s Thrush is to hook up with an expert. Which is why I recently conducted an e-mail interview with Chris Rimmer, the Director of Conservation Biology at VINS and an internationally acclaimed expert on Bicknell’s Thrush.
A Conversation With Chris Rimmer
HT: Chris, you have probably seen or captured more Bicknell’s Thrushes than anyone on the planet. Any tips for amateur birders on where to find these birds and how to identify Bicknell’s Thrush in the field?
CR: Nearly any mountaintop above 3000 feet in Vermont is likely to support at least 1 or 2 Bicknell’s Thrushes. The best bet for finding them is to climb (or drive up) one of the larger peaks like Mansfield or Killington during the month of June. If you can be there at dawn or dusk, you’ll maximize your chances of encountering the species. Once you reach the fir-spruce forest, especially the zone where trees become more stunted and windswept, start listening for the distinctive ‘beer’ call, or the wild, cascading song. That is the best identification cue. Visually distinguishing Bicknell’s from Swainson’s or Hermit, both of which can co-occur with it, requires care. Bicknell’s lack the distinctly rusty tail of the Hermit and the buffy eye-ring and face of the Swainson’s.
Right: Bicknell’s captured on Mt.Mansfield by C. Rimmer
HT: How large (or small) is the population of Bicknell’s Thrush in the U.S. and Canada anyway? Do you have any information on population trends?
CR: We can’t do much more than make educated guesses now about the total population of Bicknell’s Thrush, but we believe the species numbers no more than 20-50,000 individuals. We have not been monitoring the species long enough to have solid trend information, but we know that Bicknell’s Thrushes are declining in the White Mountains at an annual rate of 7%, while in the Green Mountains the species is declining annually by an average of 1%.
HT: In Vermont, ski areas, the timber industry, hikers and other recreational users, and now wind farm projects, not to mention environmental pollutants, all have an effect on Bicknell’s Thrush habitat. What is the impact of all this activity on the Bicknell’s Thrush and what protections are or should be in place?
CR: While individually, these projects and activities may not negatively impact Bicknell’s Thrush to a significant extent, our concern is their cumulative effects. Each ski area expansion, telecommunications tower development, or wind turbine project in montane forests inevitably removes habitat and creates human disturbance. Logging does not generally occur in these forests, but the effects of atmospheric pollution and, in the longer term, climate change, may be profound and lasting. We can not yet say what the cumulative impacts are, but we believe that direct habitat loss poses a less insidious threat than these more indirect problems. Montane areas are generally well-protected in the Northeast, and we are more concerned about atmospheric conditions that are affecting high elevation forests.
HT: You just returned from a trip to Hispaniola to assess the wintering range of Bicknell’s Thrush. What did you find there?
CR: There is no question in our minds that loss of winter habitat is putting the greatest “squeeze” on Bicknell’s Thrush during its annual cycle. We believe that 90% or more of the global wintering population of the species occurs on Hispaniola, with most birds in the Dominican Republic. Humid broadleaf forests with a dense understory are preferred, and these have been greatly reduced in extent, especially in Haiti, where < 1% of the country’s original forests remain. While a number of ‘protected’ areas exist in both countries, resources to enforce conservation are scarce, and actual protection is weak or nonexistent.
HT: Last year you reported the astonishing discovery of increased blood levels of mercury in Bicknell’s Thrush. What are your thoughts about the source of this pollutant? Is there any evidence that these mercury levels are dangerous to the survival of Bicknell’s Thrush?
CR: We do not yet have evidence that mercury concentrations in the blood and feathers of Bicknell’s Thrush are high enough to cause problems (e.g. lower reproductive success, reduced survival, altered behavior), although we are investigating this. However, our finding that levels are elevated in older birds and higher in areas where atmospheric deposition is higher, gives us cause for concern. We have little doubt that the mercury originates from atmospheric pollution, most of it from industrial sources, then bioaccumulates through the food chain, ending up in thrushes.
HT: What projects are you pursuing currently? Is there any way that birders can participate in these projects?
CR: We are continuing a long-term study of Bicknell’s Thrush ecology and demography on Stratton Mountain. 2006 will be our tenth year of intensive research on the mountain. We are actively investigating mercury burdens in the species, and trying to link mercury with other environmental stressors like depleted calcium and blood parasites. We have also been studying Bicknell’s Thrushes on East Mountain, the site of a proposed wind turbine development, hoping to assess the impact of this project, should it be permitted. We have an active research and monitoring program on Hispaniola, where we are trying to understand an apparent situation in which male and female thrushes segregate in different habitat types during winter. We are also documenting mercury burdens in winter, following up on our surprising preliminary finding that they are 2-3 times higher than on the breeding grounds. Finally, we have a long-term volunteer monitoring program, Mountain Birdwatch, which uses amateur birders to conduct annual counts of Bicknell’s Thrushes and other montane species on mountaintops all over the Northeast. We are always looking for more hardy participants. Please contact us if interested!
For further information about Mountain Birdwatch and other VINS programs, visit the VINS Conservation Biology web page at http://www.vinsweb.org/cbd/index.html.
