Last week, we stopped below the dam near the old Hathaway Mill in Waterville to see the water rushing down the mighty Kennebec River following many days of rain.

The water was spilling over the banks, and wild rapids formed where typically there’s calm water.

But what really captured our attention was what was happening in the sky. Above us swirled several masses of flying birds, chimney swifts, twittering and weaving in and out past each other, forming expanding and contracting clouds.

An artist trying to create something as exquisite as this could not have succeeded as beautifully as nature did that evening. We estimated there must have been as many as a thousand or more chimney swifts – more than either of us had ever seen in one place.

Chimney swifts are sometimes called “flying cigars,” a name that suits them well. They are small, sooty gray birds with a body the size and shape of a fat cigar.

At rest, they extend notably beyond the tail, but that’s not how you typically see them. It’s in the air that they spend most of their time, their wings sweeping back and fluttering into a blur.

Even bathing is done in the air: they swing down to the surface of the water, smack it with their breast, then lift back up to the air, shaking off the excess water from their feathers as they go.

Those who are familiar with chimney swifts are often surprised to find out that swifts are not related to swallows, even though, like swallows, they are aerial insectivores.

At one time chimney swifts nested in large, old hollow trees. Nowadays, virtually the entire population apparently nests in chimneys, smokestacks, or similar human-made structure.

They build small, half-saucer-shaped nests out of sticks. To get those sticks, they fly at twigs in the tops of trees to break them off. The birds use their sticky saliva to glue the twigs together to make the nest and stick them on the side of the structure.

Chimney swifts spend the winter in South America, though the exact range is still largely unknown. They arrive back in Maine in May.

The flock we saw in Waterville was almost certainly a migrant flock that was getting ready to spend the night in one of the old factory chimneys in the area.

Although they are seen in groups, they don’t nest that way. Nonbreeding individuals will roost together, but only a single pair nests in one chimney, which is why keeping chimneys and other structures open and available is important, especially since chimney swift populations have declined dramatically in the last four decades.

To see chimney swifts, keep your eyes and ears open next time you’re in town or other place with structures they need for breeding.

Their high-pitched twittering is a near-constant sound, and to see them, just look to the sky, where you can enjoy watching them as they provide the public service of keeping the local insect population in check.

Dr. Jeff Wells is the senior scientist for the Boreal Songbird Initiative. During his time at the famed Cornell Lab of Ornithology and as the Audubon Society's national bird conservation director, Dr. Wells earned a reputation as one of the nation's leading bird experts and conservation biologists. Jeff's grandfather, the late John Chase, was a columnist for the Boothbay Register for many years. Allison Childs Wells, also formerly of the Cornell Lab of Ornithology, is a widely published natural history writer and a senior director at the Natural Resources Council of Maine. Together, they have been writing and teaching people about birds for decades. The Maine natives are authors of the highly acclaimed book, “Maine's Favorite Birds.”