For the Birds – Bicknell’s Thrush
Maine has seven members of the thrush family among its breeding birds. These include Eastern Bluebird, American Robin and five species of thrushes with various amounts of spotting on their breast. These spot-breasted thrushes are among the finest of avian songsters. Their flute-like songs are magical, in part owing to the fact that these thrushes can control the left and right side of their syrinx, the organ that produces sound in birds. Thrushes can harmonize with themselves!
The Wood Thrush, more common to our south, but can reliably be found throughout the state. Veeries are found throughout the state. The Veery sings a song that spirals downward in pitch; the name of this thrush comes from its characteristic call note, Veer. Swainson’s Thrush, whose song spirals upward rather than downward like the Veery’s song, is a bird of spruce-fir forests. This species is therefore scarce in the southern third of Maine. Hermit Thrushes are widely distributed in the state, nesting in bogs as well as conifer forests. The final species, Bicknell’s Thrush, is the least known of our thrushes and is restricted to high elevations.
Bicknell’s Thrush has only been recognized as a full species for the past decade. It was formerly considered a subspecies of the widely distributed Gray-cheeked Thrush, which occurs from Alaska to Newfoundland in boreal habitats. Thanks to the work of the late Henri Ouellet, ofthe Canadian Museum of Nature in Ottawa, the Check-list Committee of the American Ornithologists Union elevated Bicknell’s Thrush to species status.
Ouellet built the case for Bicknell’s Thrush being a valid species with several lines of evidence. He began by demonstrating that Bicknell’s Thrushes differ in the color of the upperparts, the tail feathers, the throat and the undersides from Gray-cheeked Thrushes. Ouellet also showed that Bicknell’s Thrushes are smaller than Gray-cheeked Thrushes from the same latitudes. But morphological differences do not necessarily demonstrate that Gray-cheeked Thrushes and Bicknell’s Thrushes do not breed with each other. However, he was able to show that Bicknell’s Thrushes and Gray-cheeked Thrushes do not overlap in either nesting areas or wintering areas.
To really make his case, Ouellet needed to demonstrate that the two forms are reproductively separate. He analyzed the songs of Gray-cheeked and Bicknell’s Thrushes, finding distinct differences. Furthermore, Bicknell’s Thrushes on their breeding grounds did not respond to playbacks of Gray-cheeked Thrushes.
Ouellet’s final piece of evidence was a comparison of the DNA of the two forms. The analysis showed that the DNA differed by an amount similar to differences between other closely related species. The analysis suggested that the ancestral group split into Gray-cheeked Thrush and Bicknell’s Thrush about a million years ago.
The nesting range of Bicknell’s Thrush includes the Adirondack region of New York, and portions of Vermont, New Hampshire, Maine, Nova Scotia, New Brunswick and Quebec.
In northern New England, Bicknell’s Thrushes are generally found above 3000 feet in elevation although these birds may occur as low as 2,050 feet on some Maine peaks. Bicknell’s Thrushes like stunted spruce-fir forest, particularly areas that have been disturbed by fir waves or rime ice accumulation during the winter. They may nest along the edge of ski trails. The thick re-growth of these disturbed areas is often nearly inpenetrable, necessitating patience if you want to see one of these birds.
Male Bicknell’s Thrushes sing throughout the day until the middle of June and then singing stops abruptly. Birds will occasionally call after singing drops off but the dense habitat and secretive nature of the thrushes make them very hard to find. In short, if you want to see a Bicknell’s Thrush on its breeding habitat in Maine this year, you better plan a trip soon!
The entire population of Bicknell’s Thrushes is certainly no more than 50,000 individuals, making Bicknell’s Thrush a species of concern for environmental managers. These birds do nest at fairly high density (average of about 50 pairs per 100 acres) so habitat alteration of even a small area at high altitude can have serious repercussions for the species.
Conservationists worry about winter habitat degradation as well. The entire population winters on only four islands in the Greater Antilles.
In 1992, The Vermont Institute of Natural Sciences (VINS) launched a survey of Bicknell’s Thrushes in the northeastern United States. You can find lots of information on Bicknell’s Thrush at their website including maps of mountains with breeding Bicknell’s Thrush and downloadable recordings of songs and calls. Their URL is: http://www.vinsweb.org/cbd/mtn_birdwatch.html
You can download a copy of Henri Ouellet’s paper and two papers from the VINS research team, all published in the Wilson Bulletin, by going to: http://elibrary.unm.edu/sora/
Type in Bicknell’s Thrush in the search box. The second, third and fourth entries are the ones you will want to see.
[Originally published on June 3, 2006]