WILD NEIGHBORS: The Black Swift’s Secret

By Joe Eaton
Tuesday March 13, 2012 - 05:17:00 PM
Black swift on nest behind waterfall, Yellowdog Creek, Idaho.
Terry Gray (Wikimedia Commons)
Black swift on nest behind waterfall, Yellowdog Creek, Idaho.

Finding the breeding or wintering grounds of a migratory bird species is always a big deal. I’m old enough to recall the excitement attending the discovery of the Alaskan breeding range of the bristle-thighed curlew. Not that people were dancing in the streets, kissing random nurses, or setting fire to police cars, but there was a nice photo spread in National Geographic. (That may also have been my first encounter with the word “thigh.” In my family, in deference to my grandmother’s sensibilities, we called that part of the chicken the shortjoint.) 

A group of researchers at the Rocky Mountain Bird Observatory in Brighton, Colorado have just filled in another blank in the ornithological map. Thanks to a mix of technology and luck, we now know where the black swift spends the winter. 

I will admit that this is one of my favorite birds. They’re charismatic little guys, fond of mountains and seacliffs. Although I’ve encountered black swifts in the North Cascades, at Yosemite, and on the San Mateo coast, the best place to observe them is Burney Falls, north of Mount Lassen. The swifts nest behind the waterfall. You can watch them zoom through the curtain of water and disappear somewhere in the mossy face of the cliff. Burney Falls is also a very cool place to be on a hot midsummer day in the mountains. 

These relatively large and robust swifts have a patchy distribution from southeastern Alaska to Costa Rica. There are also populations on the larger Caribbean islands, some of which are year-round residents. Mainland black swifts migrate, though. For years no one had a clue as to their destination. 

That was typical for a bird that plays its cards close to its chest. Its nest and eggs (two at most, often just one) were not described until 1901, when A. G. Vrooman collected an egg near Santa Cruz. Nobody believed him at the time; some insisted what he had was the egg of a storm-petrel, the kind of bird you would expect to nest in a coastal cliff. It was another 92 years before the bird’s voice was recorded. No one had credibly reported a black swift sighting south of Costa Rica, much less laid hands on a banded specimen. 

A clue came in 1992 when the Colombian ornithologist Alvaro Negret collected a black swift specimen in September in a canyon of the Rio Cauca, 1800 m high in the Central Andes. It proved to be a male of the migratory subspecies borealis. Negret observed a group of 15 in that area of Colombia for a week. Then, although the canyon was still swarming with edible flying insects, they moved on, to an unknown destination somewhere in South America. 

According to an article just published in the Wilson Journal of Ornithology, Jason Beason and his colleagues cracked the case by netting four black swifts and rigging them with lightweight geolocators. These gadgets, which consist of a light sensor, a digital clock, and a memory chip, aren’t radios: you can’t use them to track a moving bird in real time. What they do is log the times of sunrises and sunsets. But if you have that much, you can calculate the longitude and latitude at which the recording was made, and you know where the device—and the bird that wears it—have been. 

There’s a catch, of course: to read a geolocator’s data, you have to recapture the bird. Fortunately, black swifts exhibit strong breeding site fidelity. Setting up mist nets near the roosts where the swifts were caught, the researchers trapped three birds that were still wearing their devices. 

If the data can be trusted, the three swifts spent the winter in Brazil, in the western Amazon basin. They seem to have covered a lot of territory, which is consistent with their foraging behavior in North America. The authors speculated that the birds may have stayed airborne for 24 hours a day, as the Eurasian common swift is known to do. Northbound, they flew west of the Pacific coast of Central America, then hung a right at Mexico and headed for Colorado. 

So now we know. It’s not exactly finding Bigfoot, but it’s a critical data point if we’re serious about protecting these birds, which are thought to be particularly vulnerable to the effects of climate change and are listed as a California Species of Special Concern (a designation more honorific than effective.) Some might lament the solving of another natural mystery. Don’t worry: there are plenty of those left.