NORWICH — Red squirrels can usually be found where northern hardwood forests meet stands of spruce and fir, habitat that enables the small mammals to stock up on their preferred food of cones and American beechnuts, among other tree seeds.

But in years when red squirrel populations soar, they roam farther, which puts songbirds that nest at higher elevations at risk, according to a new study conducted by the Vermont Center for Ecostudies and other natural science organizations that was recently in published the journal “Diversity and Distributions.”

“We’ve known for a long time that red squirrels are nest predators and we know also that mast events increase the population of small mammals,” said Dr. Michael Hallworth, a data scientist at the Hartford-based Vermont Center for Ecostudies. Mast events, which take place in the fall, are when trees produce an abundance of seeds and nuts, which many small mammals including red squirrels, gray squirrels, mice and chipmunks, feast on.

“They have higher over-winter survival and they also produce more young when there is a mast event, so the next spring there are a lot of red squirrels in the forest,” Hallworth said. But since mast events typically happen biannually, there are more squirrels competing for less food in their preferred habitat. “They’re driven by food, allowing them to expand their range.”

That causes trouble for songbirds including the Bicknell’s Thrush and Blackpoll Warbler which nest in higher elevations. Their populations have declined around 45% since 2010, according to research conducted through the Vermont Center for Ecostudies.

“The nests fail more often,” Hallworth said. “That means the red squirrels eat either their eggs or small nestlings.”

The Bicknell’s Thrush and Blackpoll Warbler are considered “threatened” species by conservation groups, due, in part, to habitat loss that is expected to increase because of climate change and logging. While mast events are not yet directly connected to climate change, when the red squirrels get into the songbirds’ territory they make an already challenging situation that much more fragile.

“They’re a natural part of the ecosystem, but the thing is we stress these systems so much that this natural relationship between predator and prey can become untenable,” said Dr. Toni Lyn Morelli, a research ecologist at United States Geological Survey Northeast Climate Adaptation Science Center and adjunct associate professor at the Department of Environmental Conservation, University of Massachusetts Amherst, who is a senior author of the study. “It can be too much given that these species are stressed by climate change and also habitat change.”

The researchers also found that the red squirrels go back to their preferred habitats the following year and songbird nests have better survival rates as a result.

“It really matters to the birds if they’re not there every other year,” Morelli said. Part of Morelli’s work involved putting tracking collars on red squirrels to see where they ended up.

The study was conducted by researchers from the United States Geological Survey; University of Massachusetts Amherst; Northeast Climate Adaptation Science Center; and National Audubon Society, according to a news release from the Vermont Center for Ecostudies, among other organizations and volunteers. Those involved checked on more than 250 camera traps from 2014 to 2019 in the Twin States — which record wildlife — and conducted cone and bird counts in a geographic area that went from Mount Katahdin in Maine, through the White and Green mountains to the Catskill and Adirondack ranges in New York State, Hallworth said.

Anne Bloomfield, a wildlife biologist in Olivebridge, N.Y., has been doing annual counts on Hunter Mountain in the Catskills each June since 2012. Bloomfield is a volunteer with Mountain Birdwatch, a citizen science program run by Dr. Jason Hill, a quantitative ecologist at the Vermont Center for Ecostudies. She hikes up Hunter Mountain the night before to get ready to count songbirds at six different locations in the early morning hours around the summit.

“Because I need near perfect conditions to conduct the survey, each year comes with a perfect breathtaking sunrise filled with the jumpy, mechanical, musical melody of Bicknell’s Thrush,” Bloomfield wrote in an email. “With Bicknell’s Thrush declining, I drink in every song and every moment. This year I was lucky to hear eight Bicknell’s Thrush singing on or between stations.”

Morelli and Hallworth stressed that volunteers played an important role in this study — and in scientific research in general.

 “A lot of this paper came out of hav  ing camera traps out,” Morelli said. “I think just being present, being aware of patterns you’re seeing can start to form the hypothesis scientists can go and test, and we can potentially use to inform conservation.”

The full study can be found at onlinelibrary.wiley.com/doi/10.1111/ddi.13861. More information about Vermont Center for Ecostudies projects — and the roles citizen scientists play — can be found at vtecostudies.org/projects.

Liz Sauchelli can be reached at esauchelli@vnews.com or 603-727-3221.