Chimney swifts screaming through dusk

The light is going out of the sky over Southampton County. The tree line holds its shape a few minutes longer than the sky above it, and in that gap, small dark birds are moving. Not gliding, not perching — moving, continuously, in wide arcs over the rooftops and treetops of Branchville. These are chimney swifts, and right now, at peak summer, they are everywhere the evening air is warm enough to hold flying insects.

Chimney swifts are built for this kind of flight. Their wings are long and narrow, set far back on a body that is almost entirely chest. Their feet are so small and weakly muscled that landing on a flat surface would strand them. Instead, they grip vertical surfaces: the rough interior walls of old brick chimneys, hollow snags, the insides of silos. They do not perch on wires or branches the way a flycatcher does. When they are not clinging inside a chimney, they are flying. That is the shape of their life. During breeding season, the adults are flying from before full light until well after dark, hunting the insects that rise into warm air columns at dawn and dusk. They catch everything on the wing: flies, beetles, flying ants, small moths. A single bird can take more than a thousand insects in a day.

Somewhere nearby, if there is an old brick chimney still standing and uncapped, a nest is attached to the interior wall. Chimney swifts build a small bracket of dead twigs, each one snapped from a branch in flight, glued together and to the wall with saliva. The nest is roughly the size of a half-grapefruit, open at the top, and it holds three to five eggs. Both parents incubate, and both feed the nestlings, which grow quickly on a diet of compressed insect pellets. By late summer the young will be flying, and by early fall the whole population will begin moving south toward South America, following the insects. But right now the nest is full and the parents are hunting.

The calls are what you notice first, usually before you pick out the birds themselves against a pale sky. Chimney swifts make a rapid, high-pitched chippering that carries well in open air — a sound that seems to come from several directions at once because the birds are moving so fast. If you hear it where you are right now, look up and slightly ahead of the sound. The birds are always a little farther along than their voices suggest. They cover a lot of ground in the time it takes to locate them. Other aerial feeders are out at this hour too — a great crested flycatcher may be making its last passes from a high snag at the wood's edge, and the Eastern wood-pewee sings its descending whistle from somewhere in the mid-canopy — but the swifts are the ones that keep moving after everything else has gone quiet and still.

What the swifts do for the ecosystem is straightforward: they take enormous quantities of flying insects out of the air every day through the breeding season. Gnats, midges, mosquitoes, and the small moths that emerge at dusk are all part of what they catch. They operate in the upper air column, in the space above where most other insectivores hunt, and they do it at the margins of day when light is low and other birds have stopped. The chimney itself, whatever building it belongs to, becomes a node in the local food web for the length of the nesting season. Old chimneys on old farmhouses and county courthouses and brick schools are the reason swifts can breed at all in a landscape where hollow old-growth trees are scarce. When those chimneys get capped or torn down, that is one less place.

The sky over Southampton County is nearly dark now. The tree line has dissolved into the general dark of the horizon. But the swifts are still up there, still calling, still moving through air that is cooling from the top down. Listen for that chippering. It is thin and fast and it comes from all directions at once.