You can hear it over the traffic on Shattuck Avenue: a high-pitched chittering, coming from somewhere overhead. Looking up, you may be able to spot a couple of small, torpedo-shaped black-and-white birds with an elegant Art Deco look, looping through the air above the downtown buildings. They’re white-throated swifts, foraging the urban canyons for airborne insects.
Swifts are odd birds. Although they have a superficial resemblance to swallows, hummingbirds are their closest relatives (the fossil record suggests hummers had swift-like ancestors). They’re built for speed, with cylindrical bodies and long, narrow wings like miniature sailplanes. They have short legs and pamprodactyl feet, the first and fourth toes capable of pivoting either forward or backward—a good design for clinging to vertical surfaces, like cave walls or cliffs.
I’ve wondered for years where the downtown swifts built their nests and only recently learned that they use the building across the street from the main library. Like most swifts, white-throats construct shelf-like structures of moss, grass, and feathers, glued together with saliva. Some of their relatives, like the edible-nest swiftlet of Southeast Asia and Indonesia, have nests that are almost pure congealed spit, the active ingredient of bird’s-nest soup. Unregulated harvesting of the nests—mostly in hard-to-reach sea caves, although one flock reportedly uses the roof of a Chinese restaurant—has caused drastic population declines in that species.
Although it’s a relatively common bird, the white-throated swift hasn’t had much attention from ornithologists. We do know that it’s highly social, nesting in colonies and sometimes gathering in flocks of over a thousand. We know that it’s migratory in part of its range, and that—again, like some hummingbirds—it can enter a torpid state when temperatures drop. We know the swift is an insect-eater, sustained by “aerial plankton” that includes both flying insects and others that drift passively on the wind. White-throated swifts have been seen following a combine harvester and scarfing the insects flushed by the machine.
But there have been few descriptions of the bird’s social behavior. Thomas Ryan and Charles Collins of California State University, Long Beach have filled some of that gap with a recent series of articles in Western Birds. Ryan and Collins observed flocks of white-throated swifts at two Southern California sites, near San Juan Capistrano and Rancho Palos Verdes, monitoring their activity patterns: arrivals at and departures from their overnight roosts. Collins, with other researchers, also intercepted the food parent swifts brought their young and analyzed the contents—mostly flies and true bugs, with a smattering of weevils and the occasional spider or silverfish. (The less common black swift was found to specialize in flying ants, a patchy but rich food source).
Back to behavior, though: Ryan and Collins describe several characteristic things white-throated swifts do, including the Courtship Fall, the Screaming Party, and the Silent Dread. “Courtship Fall” makes me think of a Butch Hancock song:
Fools fall in love
Wise men they fall too
Wise men hit the bottom
Fools just fall on through
I’ve seen Courtship Falls myself, down at Pinnacles National Monument, and they’re pretty spectacular. Here’s W. Leon Dawson, from the 1920s: “The birds come together from opposite directions, engage with the axes of their bodies held at a decided angle laterally, and begin to tumble slowly downward, turning over and over the while for several seconds, or until earth impends, whereupon they separate without further ado.” They’ve been known to plummet for 500 feet. Mating likely takes place during these falls, although it’s also been observed at the nest site.
Screaming Parties involve large groups of swifts flying past a roost site or nesting colony, all yelling their heads off. Other birds may emerge from the roost to join them. Then they break and go back to foraging. Ryan and Collins don’t provide a context for the behavior, but I have to wonder if it’s anything like the flock screams Mark Bittner has seen the cherry-headed conures of Telegraph Hill perform. Maybe it’s just avian exuberance.
Silent Dreads: Who among us hasn’t experienced those? In a Silent Dread, a group of swifts “stops calling and…departs in an uncoordinated rush, regrouping at a substantial distance from the previous center of activity.” They sometimes take place when a credible predator, a peregrine falcon or Cooper’s hawk, is nearby, although never during an actual attack. There’s a higher frequency of Silent Dreads near or after sunset, just before a flock of swifts re-enters a night roost. A swift sees something in the fading light that could be a stooping falcon and somehow, silently, communicates its panic to the whole flock.
Are these false alarms adaptive? A bird that lives such a high-velocity existence has to be able to react quickly—when a cliff looms up in the fog, when a peregrine comes at you at 200 miles an hour. That may be worth an occasional bout of the gratuitous jitters.