Atop an observation tower at the Great Meadows National Wildlife Refuge in Concord, biologist Bryan Windmiller dials a radio frequency into a receiver. He holds an antenna above his head and moves it slowly from side to side, listening for a telltale beep.
Lying dormant beneath the ice are some two dozen Blanding’s turtles, each fitted with a transmitter so researchers can track them.
For the past decade, Windmiller has overseen the Great Meadows Blanding’s Turtle Conservation Project, an effort to boost the reptile’s population. The effort involves assessing the local habitat, protecting nesting females, giving hatchlings a head start at the Stone Zoo in Stoneham and schools in several area communities, including Concord, Carlisle, and Sudbury, and then monitoring the juvenile turtles after they are returned to the wild.
Under watch this winter at Great Meadows, a sprawling wetlands preserve that covers 250 acres between Billerica and Wayland, are 11 juvenile turtles and 14 adult females that have been fitted with the small monitoring devices.
“We track the turtles in winter for two reasons,” said Windmiller, a Concord resident and head of the nonprofit Grassroots Wildlife Conservation Inc. “To find out where they overwinter, which is a distinct part of their overall habitat requirements, and to help us locate the turtles efficiently in early spring, when we need to catch many of the head-started turtles for new radios.”
Listed by the state Division of Fisheries and Wildlife as a threatened species, Blanding’s turtles face a gantlet of challenges — from loss of habitat and the risk of being crushed by automobiles to the growing numbers of raccoons and skunks, which dine on turtle eggs.
Great Meadows has the state’s third-largest population of Blanding’s turtles, behind the Oxbow National Wildlife Refuge in Devens and the Upper Parker River area around Georgetown.
But Great Meadows refuge’s turtle cohort has declined sharply since 1971, from an estimated 135 adults and older juveniles to around 60 now.
The primary goal of Windmiller’s conservation project is to help the population recover to at least the level of the 1970s.
The transmitters are glued onto the turtle’s carapace, the top part of its protective shell, and are about the size of a nickel, only thicker, for juvenile turtles, and slightly bigger than a AA battery for adults.
“There she is, she’s over that way,” said Windmiller, pointing to the frozen remains of a cattail swamp, as he picked up turtle No. 3300’s radio signal.
Windmiller said turtle 3300 was first marked in the 1980s, and is at least in her 50s.
“She’s around my age,” he quipped. “And she’s got an interesting story.”
Turtle 3300 became sick a few years ago with an upper respiratory-tract disease caused by the Mycoplasma bacteria that is marked by wheezing, nasal discharge, and eye infections, Windmiller explained. Despite being treated at the Wildlife Clinic at the Tufts School of Veterinary Medicine, she lost vision in one eye.
There are several Blanding’s turtles at Great Meadows in their 60s, Windmiller said. The oldest known Blanding’s turtle is a 73-year-old specimen in Minnesota.
Wearing snowshoes, Windmiller slowly made his way through the deep snow and a tangle of cattails, buttonbush, and swamp rose to reach the ice. As he held the antenna aloft, the signal faded in and out. This is typical in winter, he said, as the cold water reduces the signal strength of the battery-powered transmitters.
Windmiller said turtles that are brumating — the reptilian version of hibernation — spend the winter underwater, and look for well-hidden places, like root mats. They can still move about under the ice, but their bodies pretty much shut down, with very low metabolism.
Oxygen diffuses from the water into a turtle’s bloodstream through blood vessels in its mouth and cloaca, a multipurpose opening at the base of the tail used for excretion and reproduction.
Still, winter is a dangerous time for turtles, explained Windmiller. They can run out of oxygen under the ice and drown, a phenomenon called “winter kill,” especially in shallow water where there may be many organisms, such as fish and aquatic insects, competing for the limited supply available in the crowded space.
As Windmiller walked, the radio signal got stronger. “I think she’s right about here,” he said, stopping next to a small tree protruding from the ice.
He knelt down and pulled a notebook from his backpack, for recording a GPS reading of the turtle’s location and taking notes on nearby vegetation.
Each turtle is radio-tracked one time over the winter. During warm weather, researchers capture turtles three times, measuring and weighing them, taking notes on their health, and changing transmitter batteries when needed.
Another reason for radio-tracking turtles is to follow adult females to their nesting sites in spring. Some will nest in agricultural areas, like corn fields, but about two-thirds of the Great Meadows turtles nest in people’s yards.
“They tend to pick the warmest spot to dig their nests,” said Windmiller, “such as an open patch of garden or lawn next to asphalt or a sidewalk, because they retain heat and increase hatching success.
“Sometimes we knock on doors at 9 or 10 at night when we’ve found a turtle nesting in someone’s yard, and ask, ‘Do you mind if we hang out?’ Most people are pretty engaged in helping turtles, even moving their flowers so we can put a cage around the nest.”
Windmiller said the members of his conservation project put hardware-cloth screening over nests early in the season to keep out raccoons and skunks.
Eggs used to hatch in mid-September, but with warmer summers in recent years, they are now hatching in late August and early September.
Another result of hotter summers is more female hatchlings, researchers say.
Blanding’s, as well as many other turtle species, undergo temperature-dependent sex determination, where warmer nest temperatures tend to produce female offspring, and cooler temperatures produce males.
Windmiller said participants in the conservation project put screening around nests in August to protect the hatchlings and allow them to be collected for head-starting. Scarcely larger than a quarter, the tiny turtles may eventually grow as large as 10 inches long and weigh up to 3 pounds.
John Berkholtz, the senior keeper at the Stone Zoo, has been assisting the Blanding’s conservation project since 2006. Berkholtz said in an e-mail that about 10 turtles a year are given a head start at the zoo, where hatchlings from the wild are raised in a captive setting for nine months, then released.
“Head-starting increases survivorship in two major ways,” Berkholtz explained. “The increase in size . . . at release makes them too big to be easy prey for predators such as herons, bullfrogs, and raccoons. Also, they are better able to withstand changeable environmental conditions such as heat and dry conditions, and are able to locate water and food sources, especially in preparation for hibernation.”
“Mink and otters kill some of the released head starts,” Windmiller said, “and some drown. But overall, the head-start turtles have an 80 to 90 percent survival rate. Head-starting increases the survival rate about 20 times over wild hatchlings.”
Windmiller said the US Fish and Wildlife Service typically allows him to keep 50 hatchlings for head-starting, and the remainder are turned loose.
After recording the data for turtle 3300, Windmiller headed off across the open ice, following the signal of a head-started Blanding’s turtle that was released last fall.
Correction: Because of an editing error, an article in Sunday’s Globe West about a turtle conservation project in Concord misstated the area of the Great Meadows National Wildlife Refuge. It covers more than 3,800 acres.