September invariably brings mail from curious readers that goes something like this: "For the past several years on late summer evenings, I'm entertained by large flocks of chimney swifts. About a half hour before sunset, they circle in a large flock around a tall chimney at a nearby abandoned building. The flock appears to be made up of several groups of varying numbers, each flying in different patterns.
"At sunset, a few birds drop down into the chimney. The others continue to circle, as if in a holding pattern. Over the course of the next 30 minutes, more birds descend into the chimney. Eventually, the entire flock disappears.
"And a friend has told me that he has seen a similar, though opposite, flight when the birds leave the chimney each morning at sunrise."
Such letters describe late summer chimney swift behavior accurately. Here are some details of their natural history.
Chimney swifts are often described as "flying cigars" because they resemble stogies propelled by long, narrow wings. From May through September they populate the skies of towns and cities in the eastern U.S. Most winter in Peru.
Swifts nest in dark, sheltered places --chimneys, barns, silos, abandoned wells. Their ancestral nest sites, large hollow trees, are in short supply, so chimneys make ideal substitutes. And some chimney swift enthusiasts have had surprising success with man-made nesting towers.
Swift bodies are designed for life in the sky. Long, narrow wings enable rapid, agile flight. They feed by collecting flying insects in their wide, funnel-like mouths. Sources indicate that they even mate in flight.
A swift's feet are small and strong, designed for clinging tenaciously to rough, vertical surfaces. Swifts are so adapted to the flying/clinging lifestyle that they cannot perch or hop like most other birds. All four toes are directed forward for grasping onto the roost site wall. Stiff, spine-tipped tail feathers prop and stabilize their bodies on vertical surfaces.
At nesting time, chimney swifts use their strong feet to collect nesting material. They search for dead trees and limbs that have many small, brittle twigs. They fly by, grasp a twig with their feet, snap the twig off, and continue on. All this happens in a fraction of a second. Unless you know what to look for, it may seem the bird is merely catching insects in the treetops.
Twig in foot, the swift returns to its nest. At some point, either enroute or at the nest, the twig is passed to the mouth where it is coated with a heavy layer of sticky saliva. The saliva acts as a glue that cements the twigs to the chimney wall and to each other.
A shallow cup
The nest itself is a simple shelf with a shallow cup that hangs from the chimney wall. Nests are two to 20 feet below the top of the flue. Both sexes work for three to six days building the nest, which measures about four inches across. Both parents also incubate the clutch of three to six eggs for about 19 days.
The young spend about three weeks in the nest before they begin clambering about the walls of the nesting chamber. At about 30 days of age they fly out into the real world. Unlike adults, they return frequently to the nest to rest during the day.
And they sleep there at night. After being on their own for about a week, they are strong and skilled enough to stay airborne all day.
By summer's end, chimney swifts gather in flocks and roost communally for a few weeks before heading south for the winter. The chatter of hundreds of twittering swifts fills the air as they prepare to descend to the roost, like smoke strangely returning to the hearth.
UFor more information or for plans for building a free standing chimney swift nesting tower ($5), visit the North American Chimney Swift Nest Site Research Project web site at www.concentric.net/~Dwa, or send a self-addressed, stamped (55 cents) legal-sized envelope to Driftwood Wildlife Association, 1206 West 38th, Suite 1105, Austin, TX 78705.