“Stake-driver,” “thunder-pumper,” “mire-drum.” Characters in some new science-fiction-fantasy movie? No, just nicknames for a secretive bird of freshwater marshes.

The American bittern is perhaps best known for its odd vocalization, the genesis of its bizarre nicknames. The bird produces the noises deep within its throat, contorting its head and neck in the process to produce the familiar “gunk-a-lunk” calls. These guttural, low-frequency sounds can travel long distances, enabling birds that live in marshy habitats with high, thick vegetation to communicate with one another.

The bird’s eccentric habits go hand-in-hand with its vocalizations. Instead of relying on flight to escape predators, it uses camouflage to conceal itself in a most entertaining and original way. When danger threatens, it straightens and elongates its head and neck and “pretends” to be a blade of grass, a twig or a cattail stem. It takes the pretense a step further by actually swaying in the breeze along with the vegetation.

The bittern is a long-toed wading bird that stalks the heart of marsh and meadow for its favorite prey. This doesn’t mean it won’t resort to swimming, however, as it did when Fran Graham of the Bangor Nature Club saw one. Graham and fellow club member Pearl Jarvis were in the middle of their bird-a-thon and had stopped at the Fields Pond Nature Center in Holden. In the frog pond nearby they saw a male hooded merganser and another bird they couldn’t readily identify.

“We were just about to leave,” Graham said, “when a light tan, ducklike bird swam really slowly across the pond. It had a dark brown streak going down the side of its neck.”

Now, the first thing I thought upon hearing this was that it could have been a female hooded merganser. But this bird’s profile is very distinctive, hard to mistake for anything else; and the female does not have any dark streaking on her neck – it is a uniform earth-brown color throughout. Plus, Fran and Pearl had searched their bird books and could find no bird with webbed feet (such as the hooded merganser has – and the bittern does not) that matched the appearance of the bird in the pond.

And an American bittern is known to frequent that pond, which probably is full of tadpoles that it loves to eat.

Fran began to investigate. First she talked to Judy Markowksy, the center’s director, who then searched the scientific literature for some mention of this behavior to no avail. Next she talked to Ron Joseph, a biologist with the U.S. Fish and Wildlife Service, and was gratified to hear that he, too, had seen an American bittern swimming.

The American bittern is seldom seen, so it is not surprising that many of its behaviors still are unknown or unfamiliar. This is part of what makes birding so challenging and so much fun – especially when you’re dealing with a mysterious marsh sprite like the bittern.

Chris Corio, a volunteer at Fields Pond Nature Center in Holden, can be reached at fieldspond@juno.com