Volunteers Jerry Smith and John Wyatt led 10 birders on an Audubon field trip last week, from Bangor to Belfast along the river.

In Belfast Harbor, the highlight was seeing eight Barrow’s goldeneyes, a rare species which is now the center of controversy about whether it should be on the Maine Endangered Species list.

Maine Audubon is of the opinion that it should be on the list, and the birders were delighted to see such a rare and beautiful species.

Another highlight of the trip was a Cooper’s hawk sitting in a tree near the Bangor Auditorium. The leaders were able to show the others how a Cooper’s Hawk is identified: by its long tail, short wings and black cap. The leaders also explained why the hawk was sitting there in the city. It was keeping its eye on the big flocks of pigeons and starlings near the auditorium, watching for one that was slower than the rest, an easy catch.

An unexpected highlight was seeing more than 100 mallards, a common duck that most birders take for granted. However, these 100 mallards were swimming in a large sediment settling tank at the Bangor Waste Water Treatment Plant.

The birders wondered what the ducks were doing there. Did the tank hold the only water around that was not frozen? Was it warm enough to warm their feet? Were they there only to swim around? Or, could it possibly be that they were – um – feeding?

I called Tom Hambrock, who is the laboratory manager at the treatment plant, knowing that he is a good birder. He watches and photographs birds up and down the Penobscot River. He was very informative.

He confirmed that the mallards were indeed feeding in the top of the settling tanks. He knows that because he has often seen them swinging their bills in an arc in the water, vibrating their bills open and shut at high speed.

I got up my courage to ask him what he thought they were eating.

“Bristle worms and manure worms. Some rise to the surface of the settling tanks. They have the appearance of miniature earthworms,” he said. “The settling tanks never freeze, so they provide open water for the ducks during the coldest winter months.”

So, these worms and other small organisms in the tank, with the help of aeration, do their work of turning sewage into clean water that goes into the Penobscot.

Brad Moore, the manager of the plant, explained that the treatment plant is operated in such a way as to provide the right environment for the small organisms to thrive, reproduce and clean the water – no mean achievement. Kudos to the treatment plant folks for all they do.

For information on Fields Pond Audubon Center, call 989-2591.