Chimney swifts make me smile. Even the chattering sound they make overhead gives me a grin. I always hear them before I see them, because they simply can’t help themselves. They must chatter.

Swifts are an ancient family of birds, diverging from the rest of the avian world perhaps 40 million years ago. The Latin name for the family is Apodidae, which comes from a Greek word meaning footless. And footless is exactly how they appear. They live on the wing and cannot perch as other birds do. They have tiny feet but long claws, which are designed to grasp vertical surfaces, such as the inside of a chimney or upright hollow log. Swifts also have short, stiff tails that allow them to brace on a vertical surface.

A hundred species of swift span the globe, but there are only four species in North America, and only the chimney swift inhabits the East.

All swifts feed on the wing, snatching anything that flies — even tiny spiders clinging to webs drifting in the wind. Swifts have tiny bills, but they can open them exceptionally wide as they vacuum insects out of the air. Although they can’t land on a branch, they are nimble enough to pick a bug off a leaf while swooping by.

Swifts are most closely related to hummingbirds, and they share a noteworthy characteristic. Swifts and hummingbirds can rotate their wings at the base. It’s a trait that allows hummingbirds to fly backward. It allows swifts to be exceptionally maneuverable in the air.

In addition, swifts have thicker bones in the wingtip compared to most other birds, and the forearm is stronger. This allows them to reshape their wings in flight. Combined with the ability to rotate the wing, swifts are among the fastest, nimblest birds in the air. They are so agile that they even bathe on the wing, splashing lightly into the surface of a pond and shaking loose the excess water as they rise.

As the name implies, swifts are fast. Their long, narrow wings are tapered at the end — a feature seen in all birds built for speed. The swift’s ability to rotate and shape its wing gives the bird an odd looking flight, like a flying cigar zig-zagging above the rooftops. Often, it appears the wings are beating in alternating fashion. They’re not. It just looks that way. Their tails are too short to use as a rudder, so they turn quickly by flapping harder on one side while pulling the other wing closer to the body. This creates the illusion of alternating wingbeats.

Chimney swifts evolved to nest on cliff faces, cave walls and inside hollow trees, but when Europeans brought chimneys to America, swifts quickly adopted them. Some swifts still nest in trees, and I often find them deep in the Maine woods, far from civilization. Swifts produce a sticky saliva that they use to glue twigs together into a nest and cement it to the vertical surface.

Chimney swifts are monogamous and generally mate for life. Some swift species actually mate in the air, but chimney swifts are content to copulate while clinging to a wall somewhere near the nest. They typically produce a clutch of four to five eggs, but up to seven eggs have been recorded in some nests.

Swifts are not colonial nesters. It’s one pair per chimney. But unmated swifts often congregate in a large chimney, as do mated swifts after the breeding season. Years ago, right at sunset, I marveled at a huge cloud of swifts disappearing down the chimney at the consolidated school in Greenville. It looked like a swirling vortex of smoke being sucked back down the chimney.

That’s a phenomenon that is less likely these days. Chimney swift populations have declined by 75 percent since studies began in the 1960s. Many large chimneys have been demolished and small chimneys are usually capped. In the woods, forestry operations have curtailed the number of standing dead trees. On the plus side, some communities where swifts historically mass during migration have erected swift-roosting towers, with mixed success.

Many songbirds migrate at night, but the fastest, nimblest flyers — those that can avoid hawks — go in daylight. Chimney swifts migrate by day in small flocks, high in the air, all the way to South America. They’ve been recorded at altitudes exceeding a mile.

Most of Maine’s larger communities have chimney swifts right now. Bangor has loads of them. Look up, listen and smile.

Bob Duchesne serves as vice president of Maine Audubon’s Penobscot Valley Chapter. He developed the Maine Birding Trail, with information at mainebirdingtrail.com. He can be reached at duchesne@midmaine.com.