It's about 9 o'clock on a Friday night and three people are standing in the dark by Puffer Lake near the Maynard-Sudbury line, listening to the racket.
To the untrained ear, the noises are simply loud and vaguely familiar -- the twang of a string, a low grinding that sounds like an alarm-clock buzzer on a weak battery, an occasional cheep-cheep-cheep.
But Paul Scheiner, Debbie Dineen, and Mary Drake are listening to various species of male frogs -- including the green frog, gray tree frog, bullfrog, and peeper -- trying to distinguish themselves from competitors in a marshland mating game.
For the frogs, it's about reproducing. For the researchers, it's mostly about fun. "I'd rather go out in the woods listening to frogs for a few hours than to sit at home channel-surfing on a dull Wednesday night," said Scheiner, a Maynard resident who is coordinating the frog call survey.
But the three volunteers are seeking more than entertainment: They're conducting research as part of a nationwide effort to track populations of individual species of frogs and toads.
Researchers can't count all the frogs, but at each stop they determine in rough terms how many of each species they are hearing -- whether it's individual frog calls, frog calls that occasionally overlap, or a chorus of calls all happening at the same time. The rating system offers an idea of how many frogs are in a particular area.
The effort was launched after scientists began noticing a worldwide decline in amphibians in the late 1980s, said Scott Jackson, Massachusetts coordinator of the North American Amphibian Monitoring Program, a related effort.
Possible factors in the decline of the amphibian population include pollution from industrial sites and chemicals used in agriculture, diseases caused by fungus or virus, and habitat destruction from development or agriculture, Jackson said.
"It's not across the board, but it's widespread, and it's somewhat mysterious," said Jackson, program director for natural resources and environmental conservation at the University of Massachusetts at Amherst.
Because frog populations go up and down significantly based on natural factors such as rainfall, statisticians estimate they need about 15 years of data before they can confidently discern trends. Jackson described the frog survey as "an early-warning system" to alert scientists to trends in population.
"If it starts to decline, we want to catch it early," he said, "but we also want to make sure that we don't cry wolf and spend a lot of time and resources on something that isn't a problem."
Nationally, field surveys began in earnest in 1997. Biologists at the federally owned Eastern Massachusetts National Wildlife Refuge Complex began annual spring surveys of frog calls in 2000.
But by last year, federal officials in this area had decided to give it up because of a lack of manpower, focusing instead on their breeding bird survey, said Stephanie Koch, a wildlife biologist for the US Fish and Wildlife Service.
That's when local volunteers stepped in, agreeing to take over the frog-call survey. The biologists trained the volunteers to follow strict protocols governing time of day, time of year, listening time, and temperature, so the results can be compared no matter where they are gathered.
Certain species of frogs are most active within certain temperature ranges, so the surveys take place at several wildlife refuges during four windows (each about 10 days long) from early spring to early summer. Because frogs tend to call at night, the surveys must begin about a half-hour after sunset. The local volunteers' route at the Assabet River National Wildlife Refuge, which includes portions of Sudbury, Stow, Maynard, and Hudson, has about 15 stops and can take three hours or more.
Since the frogs call in numbers only when the weather is right, the survey requires participants to be flexible; plans to make the rounds are often finalized as late as the afternoon of the night the group goes out.
The survey also has some auxiliary benefits. Because frogs breathe through their skin, scientists believe they may be particularly susceptible to changes in the environment, such as air or water quality. So changes in frog populations may provide broader clues about how the environment is doing.
"Because it is an indicator of wetland health, we're really happy to see it continue," Koch said. "It's one of those things that would not have happened without the volunteers."
The volunteers are aware of the serious research they are participating in, but they mostly consider their outings to be recreation. "I love the wildlife," said Drake, also of Sudbury. "I'm hoping to see something different, and the people are fun. You never know what you might find in the middle of the night in the middle of the refuge."