This story by Aaron Calvin was first published in the Stowe Reporter on July 31.
Despite an increasingly threatened habitat and dwindling population, a rare songbird in its Mt. Mansfield habitat saw the return of many adults during its annual banding this year — a sign of the bird’s endurance.
The Vermont Center for Ecostudies has been studying the Bicknell’s thrush — a small branch-brown songbird, named for the amateur ornithologist who first identified them in the Catskills — for over three decades.
Earlier this month, the Center, under lead bander Anna Peel, once again set up nets to capture and tag the thrush with tracking devices during its mating season. The Center monitors the bird population to learn more about its migration patterns and how well it survives year to year, as well as to count the number adult and young birds on the mountain.
The Bicknell’s thrush is picky about their habitat. They spend their mating season in the northeastern United States and Canada, with a preference for “sky islands” like the alpine summit of Mansfield — though most of the population winters almost exclusively on the Caribbean island of Hispaniola.
The thrush, along with their subtle coloring, trilling song — which they prefer to sing at sunset instead of sunrise — display unique mating habits, just one of the mysteries that still abound with this songbird, according to Desiree Narango, a conservation scientist with the Center who specializes in the songbird.
Based on data that skews heavily male, as the females are harder to catch, Narango said the birds exhibit polygynandry, meaning females partner with multiple males and males partner with multiple females in the nesting process.
“They have a really interesting population dynamic, and that sex ratio changes from year to year, so in some years we have a tremendous number of males and a very low number of females, and in some years it’s more even,” Narango said.
The difficulty in catching females makes learning more about the birds’ private lives challenging, and in particular has prevented researchers from gleaning much about the females, a gap in knowledge that is essential to the challenged species’ survival.
The warming world caused by the ongoing climate crisis threatens all alpine bird species, but the already scarce Bicknell’s thrush population is particularly vulnerable. Data collected through the Center’s crowdsourced collection program Mountain Birdwatch indicates that Bicknell’s thrush populations have declined by an average of 5% per year in Northern New England and Eastern New York over the last 15 years.
The Mt. Mansfield banding project began over 30 years ago with a specific focus on the Bicknell’s thrush and has expanded over time to track six different Alpine bird species. Two of those species — the white-throated sparrow and dark-eyed junco — are among the fastest declining bird species in North America, according to Narango. Exactly why remains an open question, but researchers broadly point to human-caused environmental pressures, including climate change.
“Part of our work right now is trying to understand what those drivers are of (Bicknell’s thrush) females, because that’s ultimately changing the population,” Narango said. “Another part that we’re currently trying to better understand is what their diets are and what kind of insects that they might be more closely reliant on to feed their young and to sustain the population.”
Still, at this year’s banding, Narango and her colleagues observed at least one positive population signal.
“We actually had a lot of returning birds this year, so birds that we’ve captured in previous years have come back, meaning that they’ve survived over the season, which is good news for the population, but also provide some information to us that the events that happened during other times in the cycle, such as migration or during the winter, were favorable for first survival in this species,” Narango said.
