\nBerkeley Dayly Planet
\nBy JOE EATON Special to the Planet<\/p>\n
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\u2019re white-throated swifts, foraging the urban canyons for airborne insects.<\/p>\n
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\u2019re 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\u2014a good design for clinging to vertical surfaces, like cave walls or cliffs.<\/p>\n
I\u2019ve 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\u2019s-nest soup. Unregulated harvesting of the nests\u2014mostly in hard-to-reach sea caves, although one flock reportedly uses the roof of a Chinese restaurant\u2014has caused drastic population declines in that species.<\/p>\n
Although it\u2019s a relatively common bird, the white-throated swift hasn\u2019t had much attention from ornithologists. We do know that it\u2019s highly social, nesting in colonies and sometimes gathering in flocks of over a thousand. We know that it\u2019s migratory in part of its range, and that\u2014again, like some hummingbirds\u2014it can enter a torpid state when temperatures drop. We know the swift is an insect-eater, sustained by \u201caerial plankton\u201d 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.<\/p>\n
But there have been few descriptions of the bird\u2019s 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\u2014mostly 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).<\/p>\n
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. \u201cCourtship Fall\u201d makes me think of a Butch Hancock song:<\/p>\n
Fools fall in love<\/p>\n
Wise men they fall too<\/p>\n
Wise men hit the bottom<\/p>\n
Fools just fall on through<\/p>\n
I\u2019ve seen Courtship Falls myself, down at Pinnacles National Monument, and they\u2019re pretty spectacular. Here\u2019s W. Leon Dawson, from the 1920s: \u201cThe 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.\u201d They\u2019ve been known to plummet for 500 feet. Mating likely takes place during these falls, although it\u2019s also been observed at the nest site.<\/p>\n
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\u2019t provide a context for the behavior, but I have to wonder if it\u2019s anything like the flock screams Mark Bittner has seen the cherry-headed conures of Telegraph Hill perform. Maybe it\u2019s just avian exuberance.<\/p>\n
Silent Dreads: Who among us hasn\u2019t experienced those? In a Silent Dread, a group of swifts \u201cstops calling and\u2026departs in an uncoordinated rush, regrouping at a substantial distance from the previous center of activity.\u201d They sometimes take place when a credible predator, a peregrine falcon or Cooper\u2019s hawk, is nearby, although never during an actual attack. There\u2019s 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.<\/p>\n
Are these false alarms adaptive? A bird that lives such a high-velocity existence has to be able to react quickly\u2014when 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.<\/p>\n","protected":false},"excerpt":{"rendered":"
Berkeley Dayly Planet By JOE EATON Special to the Planet 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 […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":[],"categories":[413],"tags":[],"yoast_head":"\n