The chittering birds that own the air and rarely leave it

By BILL CHAISSON
Of a Feather
When we moved into our house in an old mill town in the Hudson Valley, it was apparent that the old couple who had lived there had not used the fireplace in several years. The brick chimney stood high above the roof and it was very wide at the top to accommodate two separate flues for hearths on the first and second floors. In the evenings I began to watch a tribe of chittering birds that tore across the sky on stiff scimitar-shaped wings. They beat their wings like wind-up toys, which distinguished them from the fluid motion of swallows. The wings of swallows are slight bent like chevrons and their wing beats are deeper and their flight more acrobatic.

I don’t remember the first time I saw one of these birds drop down into the chimney, but that is when they displayed their acrobatic prowess. They seemed to suddenly collapse in mid-flight and fall in a tight fluttering spiral into the opening. They were chimney swifts, and they were dropping into their nests, which were glued to the inside walls of the chimney.

We have only one swift species in eastern North America, but there are two more out West and another two that occasionally wander north to the United States. I saw one of the latter two while traveling in Chiapas, Mexico near the Guatemala border. We took a day trip to a swimming hole called Agua Azul. This is a karst terrain, which means the limestone bedrock is constantly being dissolved in the rainy season, transported in surface or ground water and then redeposited elsewhere during the dry season. Consequently the rivers become a series of pools lined by grayish-white rims that look like they have been built by children making an elaborate sand fort.

Soon after our arrival I noticed the large dark birds dashing over the river. Unlike chimney swifts, which generally stay well above the rooftops and trees, these birds — which I later learned were called white-collared swifts — swooped low over the water, emitting throaty calls constantly. Sometimes you could even hear the rattle of their feathers as they banked and abruptly upped the g-forces on their bodies.

These swifts were much larger than chimney swifts, nearly 9 inches long compared to 5.5 inches, with wingspans up to 21 inches versus 12 inches. But I experienced a very similar level of surprise when I realized where their nests were. The primary cataract at Agua Azul is probably 30 to 40 feet high. The Usumacinto River was at a low ebb at the end of the dry season, but it was still an impressive roar. The swifts flew directly through the plummeting river to get to their nests behind the falls and then they popped out again to resume hunting for insects for their young.

There are 100 species of swift around the world. They are thought to be most closely related to hummingbirds, with which they share short, small, and relatively weak feet. This latter characteristic gives the family its name Apodidae, “without feet.” Because swifts are constantly on the wing, Europeans were under the impression that they lacked feet altogether and depicted them that way in medieval heraldry. Although they do have feet, they are incapable of perching, but can only cling to vertical surfaces or, of course, sit on their nests.

Chimney swifts, as their name suggests, have adapted to human structures. The similar-looking Vaux’s swift of northwestern U.S. and southwestern Canada has retained its fondness for natural settings, attaching its nest to the inside of hollow trees and in sheltered places on rock faces. It has only recently begun to choose built structures as nest sites.

Most swifts use various materials to built their half-cup shaped nests, holding everything together with their saliva and also attaching it to the site surface that way. The swiftlets of the genus Aerodramus famously make their nests entirely out of saliva, and they have been the eponymous ingredient for bird’s nest soup in China for 400 years.

Like many insect-eating birds, the chimney swift population has been in a long-term decline. Between 1966 and 2015 their numbers decreased by 2.5% per year for a cumulative reduction of 72%. That said, there are still 18 million of them in the world. The advent of European settlement and the construction of many, many chimneys in all likelihood swelled the numbers of this species through the 18th and 19th centuries. The advent of widespread pesticide use in the the 20th century is thought to have caused the initial decline.

However, the species’ fondness for chimneys as nesting sites is now working against it. Architectural styles changed through the 20th century and large open chimneys have become much less common. The chimneys of older buildings are increasingly poorly maintained and have become unsuitable nesting sites; many of them have also been capped, preventing swift entry altogether. Finally, some people are unaware of the presence of the swift nests or object to them and have their chimneys cleaned during the nesting season.

It would be difficult for chimney swifts to return to their pre-settlement nesting sites in large old hollow trees, as these are relatively rare now because of modern scientific logging methods.

Anyone wishing to cultivate a swift colony can leave their chimney uncapped and have it cleaned outside the breeding season. If you don’t have a chimney you can build a breeding tower. The specifications are available through chimneyswifts.org.

In 1928 amateur ornithologist Althea Sherman of Iowa had a nesting tower built with peepholes so she could observe the nesting habits of the swifts. Her tower immediately attracted the birds and she watched them for over 15 years and accumulated 400 pages of notes.