Rigor Vitae: Life Unyielding

Sunday, May 28, 2023


 This is the time of year when one of our most mysterious bird species returns to our area from its wintering grounds. With an 18" wingspan, the Black Swift is significantly bigger than other North American swift species. It was never common, but over the past half century, its overall population seems to have decreased by about 90 percent. It looks very much like a larger version of the much more common White-throated Swift, only its plumage is completely blackish. Some individuals have whitish tips to the feathers of their undersides, and for a long time this was presumed to be a characteristic of juvenile birds. A recent study found that it tends to be a mark of female birds of all ages, although it's not a dependable way to distinguish the sexes, as some females lack the white tips and some males have them. The white tipping also tends to be more distinct in juvenile birds. The sexes can be reliably told apart by the shape of their tails; those of males are distinctly notched, while the females' tails are not. The only place I know of in Utah where Black Swifts reliably nest is Mount Timpanogos. When you hike to the top of that mountain in the summer, you can usually spot one or two flying about high in the sky, if you keep your eyes open. Black Swifts have also been recorded in Iron, Kane, Sevier, Washington, Cache, Salt Lake, Duchesne, Uintah and Wasatch Counties. Like other swifts, they spend all day aloft, even sleeping while flying, in a technique known as "aerial roosting." Normally, they only land to nest. A crevice in a rock face, usually behind a waterfall, is selected as a nest site. A single egg is laid, and both parents care for the young bird. They bear a throat pouch that they can fill up with insects caught on the wing. Once the young bird is flying well, in about mid-August, the family disappears. For a long time, nobody knew where they migrated to, until a couple of years ago, when researchers in Colorado fitted four Black Swifts with geolocators and managed to retrieve three of them the following year. All three birds had followed roughly the same route to winter in western Brazil. What sort of habitat they exploit or how they live is still unknown, since there are still no documented sight records of Black Swifts in Brazil. We only know that they make it back up to our region around the end of May. The IUCN lists the Black Swift as "vulnerable," with an estimated population of 170,000 mature individuals.