I never know what to expect when I visit the Penjajawoc Marsh in Bangor, but one thing is guaranteed: I am always pleasantly surprised.

This was especially so when we did our Birdathon three weeks ago. We went there in the hopes of adding particular birds to our list, and in many cases we weren’t disappointed. This was the place where we expected to identify the greatest number of birds; then we’d be off, traveling to other areas to get birds not found this far inland.

But even in the middle of the time-intense, count-’em-and-run event, we were lulled into just stopping and appreciating what was around us.

This was especially so when we were taken by surprise by the appearance of a black tern.

We had known that a couple of these birds had nested in this marsh in previous years. We didn’t expect to see one by chance; we didn’t know if they had returned, or where they would decide to nest in this large wetland.

The bird came fluttering high over the center of the marsh on delicate, tapered wings. Its flight was so buoyant it seemed as if it were lighter than the air itself. We watched as it swooped effortlessly, deftly catching insects on the wing. Its dark form remained visible as it flew along the wetland, after which it turned to make another pass.

Arcing, tilting and banking, it made flight an art form. We only stopped watching when it finally disappeared into the distance.

The black tern is the rarest species of tern to be found in Maine, according to the Department of Inland Fisheries and Wildlife. It is listed as endangered in the state, and is considered a species of regional concern in the Northeast because it is only found in two other Northeast states – New York and Vermont.

This is so, in part, because of loss of the species’ habitat. Unlike other terns, which choose rocky islands off the coast to nest and catch fish in the ocean, black terns inhabit freshwater locations. These environments have been, and still continue to be, destroyed and degraded at an alarming rate. According to “The Birds of North America,” more than half of the wetlands in the contiguous United States have been destroyed since we settled this country.

Terns – as well as some other water birds – prefer to nest in large bodies of shallow water. The other requirement is that these locales must be interspersed with vegetation such as cattails, sedges, and rushes. This helps protect their flimsy nests from wind, waves, and wakes from boats. It also provides a barrier to land predators.

Black terns differ from other terns in another respect: insects, not fish, make up the majority of their diet. Because of this, they may forage over land as well as water.

Black terns are not completely black. Rather, they are more a light-gray, sooty color, with dark heads and beautiful silver linings along the edges of their wings. In the winter they acquire a lighter plumage.

In the future I hope to see more of this “restless waif of the air,” as scientist A.C. Bent, quoted in the BNA, described it. It’s a sight not to be missed.

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