Over the past two summers, visitors to Walden Pond in Concord have seen and heard something unusual at the body of water made famous by Henry David Thoreau in his book “Walden.”
“We have common loons at Walden Pond for the second straight summer,” said Peter Alden, naturalist, author, and co-founder of Spark Birding, a local bird-watching organization. “It’s new and exciting to hear them at Walden.”
Alden, who also works at the Thoreau Society Shop at Walden Pond, said that in summer of 2020 there were frequent sightings of a solitary adult loon and often a pair of loons. This year, a pair was involved in courtship “dancing” in May.
“In recent weeks there has often been a single loon at Walden at all times of day, even calling at mid-day,” said Alden. “That raises suspicions that perhaps there is a nest somewhere nearby and whichever adult is not at nest will fish at Walden.”
The common loon, with its black head and distinctive white and black checkerboard back, and eerie, mournful call, is a summer visitor at lakes from northern New England to Alaska. But it’s not common in Massachusetts, especially in the eastern part of the state.
“In Ludlow Griscom’s ‘Birds of Concord’ , he states that loons nested in lakes near Boston up until 1830,” said Alden. “Having pairs of loons summering at Walden and nearby lakes is likely the first time in 200 years that breeding here again may be on the horizon.”
In an article about loons in a 2018 edition of “Massachusetts Wildlife” magazine, author Deborah McKew wrote that common loons were found throughout New England until the 1800s.
“However, as humans shifted from a culture of subsistence hunting to shooting for sport and built encroaching houses and camps on critical shoreline nesting habitats loon populations suffered serious declines. Over the course of a century, the bird’s southern range was greatly reduced … By the late 1800s, the haunting calls of the loon were no longer heard on Massachusetts’ lakes and ponds.”
The last known breeding pair of loons in Massachusetts was reported in 1872, McKew wrote.
“It would take nearly 100 years  before a loon pair had established a territory — breeding and nesting at Quabbin Reservoir. Since then, loons have ever so slowly begun to repopulate a portion of their former range.”
Marion Larson, Chief of Information and Education for MassWildlife, wrote in an e-mail that statewide monitoring of nesting loons, which are listed as a Species of Special Concern in Massachusetts, is a collaborative effort among the Massachusetts Division of Fisheries and Wildlife (MassWildlife), Massachusetts Department of Conservation and Recreation (DCR), and Biodiversity Research Institute (BRI), a nonprofit ecological research group.
Andrew Vitz, State Ornithologist for MassWildlife, wrote in an e-mail that during the 2020 nesting season 48 territorial loon pairs were documented producing 30 nests, 24 hatchlings, and 19 successful fledglings.
Vitz said the highest number of territorial pairs was documented on Quabbin Reservoir (18), followed by Wachusett Reservoir (5). These pairs were monitored by the state Department of Conservation and Recreation. In suitable nesting habitat not monitored by DCR — smaller waterbodies primarily located in north-central Massachusetts —MassWildlife and BRI conducted surveys for common loons and deployed four loon-nesting rafts.
Larson said MassWildlife is also working on a multi-year restoration effort involving the capture and translocation of young loons to southeastern Massachusetts.
“Last year, a historic loon hatch occurred in the southeastern part of the state in early July,” said Larson. “What was exciting is that one of the birds was a relocated loon!”
Alden said common loons grow to about 32 inches long, with a wingspan of 46 inches, and can weigh 10 pounds. Their legs are so far back on their body that they can’t walk well on land, so they need big ponds to act as runways to take off.
The Cornell University website All About Birds said loons need from 30 yards to a quarter-mile to gain enough speed for liftoff, and may become stranded on ponds that are too small. Once airborne, however, they are fast fliers, and have been clocked at speeds of over 70 miles per hour.
The common loon is a quick and agile swimmer and propels itself underwater with its legs as it pursues fish, according to the Cornell website. Vitz said common loons forage on a variety of small fish, crayfish, insects, crabs, and snails.
Loons tend to nest in quiet, protected spots on lakeshores, the Cornell website said. Because they can’t walk well on land, nests are usually built close to a bank, where the water is deep enough to allow the loons to approach the nest from underwater. Loons will sometimes use artificial nesting platforms.
The nesting platforms are floating rafts, which protect loon nests from raccoons and other predators that might eat eggs or hatchlings, Alden explained. The platforms are anchored to the lake bottom so they won’t float to shore. They can have soil and plants on them to make them appear like natural islands.
“Loon rafts can be a very important tool, but their benefits really depend on the specific waterbody,” Vitz said. “Rafts are most important on waterbodies that regularly have fluctuating water levels. Loons nest very close to the water’s edge, and rising waters can drown nests and lowering water can cause loons to abandon nests.”
Loon rafts have canopies, primarily designed to offer protection from aerial predators such as eagles, explained Vitz. Canopies also provide shade on hot summer days, he added, which may become increasingly important with warming temperatures associated with climate change.
Common loons typically lay one to two eggs in spring or summer, and the young are able to ride on their parents’ backs and swim within hours of hatching, according to the Cornell website.
Juvenile loons are on their own after about 12 weeks when their parents migrate to coastal waters for the winter, where they shed their summer colors for gray and white plumage. The young spend two years on the coast before returning to lakes in their third year, although they may not breed until they’re about six years old. The oldest recorded common loon was a 29-year-old female.
The number of loons is going up, said Alden, due in part to conservation efforts.
“It’s nice to know a big, iconic bird is doing well due to actions by humans.”
Don Lyman is a biologist, freelance science journalist, and hospital pharmacist who lives north of Boston. Send your questions about nature and wildlife in the suburbs to firstname.lastname@example.org.