WISCASSET, Maine - After months of bitter cold, we have at last reached that time of year when nature starts to wake up. Some of the first - and most audible - signs of spring's arrival are the mating calls of amphibians - peepers and wood frogs.
It's also the time of year when Maine's citizen scientists go to work, clipboards in hand, trying to keep track of just how much "action" is going on out there.
Every April, Michael Hudson goes looking for frogs - or to be more accurate, goes listening for them. This is his twelfth year as a volunteer for Maine Audubon. It's after sunset and the creatures he's hoping to hear are between 1 and 3 inches long.
"You rate your observation based on whether you hear a few single individuals peeping, where you might be able to count the individuals," he says, "and that would give you a 'one' code."
A "two" code, he explains, would be when he hears overlapping calls - too many to tell the exact number of frogs, but short of a full-throated chorus. "If it's a full chorus we'll give it a 'three' code, so basically it's a three-pronged, three-code system.
On this mid-April night, we're looking for wood frogs and spring peepers who are active this time of year in their hunt for a mate. We're at the first of 10 pre-planned sites in mid-coast Maine near Wiscasset, and Hudson hears something emanating from a vernal pool a few yards from where we've parked.
"I can hear them in the distance right now and I'm going to walk up the road a few feet just to get a better listen," he says, "because there's a little bit of a rise to the road and maybe I can hear a little bit more."
As we approach the edge of a fenced-off field, the sound becomes louder. They're wood frogs, says Hudson. But to the untrained ear, they sound like a completely different animal.
"They sound like ducks," I say.
"Yeah, they're just like ducks feeding in the puddles," Hudson says.
I ask Hudson how he'll rate this observation. "I'd give that about a 3. I'd give it a 3," he says.
"So approaching a full chorus?"
"Approaching a full chorus, yeah. It waxes and wanes, but there are periods here where they are just constant."
What Michael Hudson is doing in Maine is being replicated by volunteers in 24 states across the country. Susan Gallo is a biologist with Maine Audubon, which established the Maine Amphibian Monitoring Program in 1997.
"This is definitely the latest season we've ever had," Gallo says. Scientists like Gallo will be anxious to know if this year's data follow a disturbing trend highlighted in a report issued last fall. It was based on 10 years worth of surveys across the Northeast, between 2001 and 2011.
"All the species, except one, in Maine are declining," Gallo says. Spring peepers and wood frogs, she says, appear to be dwindling while the northern leopard frog is now on the group's "special concern" list. Its estimated numbers have dropped by about two-thirds.
The report, issued by the U.S. Geological Survey, does not suggest any factors behind these declines. But Susan Gallo suspects encroaching development near vernal pools may have played a role.
Maine has now taken steps to protect vernal pools, where wood frogs breed and lay eggs. But there's also concern about a new fungus that affects amphibians, causing a potentially lethal skin disease.
And there's speculation, too, about the role of climate change. "If invertebrate populations are changing, if lake temperatures are changing, you know, how is the whole food chain for frogs affected?" Gallo says. "And I don't know of anyone who has an answer to that, but we know things are changing."
Back on the survey, we're now near a stream at location number two, and the peepers are out and calling - but not that many of them. Hudson rates their level of activity as a two. Among the mating calls is this curious sound (bleating frog sound). Hudson explains what it means.
"Sometimes when they get too close to each other they do a bit of an aggressive call, to say, 'Get out of my space, you're getting too close,' " he says.
"Like humans aren't they?" I joke.
"Yeah," he laughs.
"What do you get of this the most?" I ask.
"I just love being out on a spring evening like this, and it's my contribution to citizen science," Hudson says.
This is the first of three amphibian surveys done every year. Towards the end of next month, Hudson will be out again, looking for leopard frogs and pickerel frogs, as well as some toads.
And in June, he'll be scouting for the big-throated sound of bullfrogs, green frogs and tree frogs, without whose night-time chorus those spring and summer nights in Maine would not be the same.