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When we arrived at the industrial site in Marin County (and that’s as specific as I can get) around 6:15 pm last Monday, a few swifts were already circling the tallest of three chimneys. Berkeley birder Rusty Scalf discovered this roost of migrating Vaux’s swifts in late September. I had assumed that the birds had subsequently moved on south, but friends had reported their continuing presence the day before. Since we had other business in Marin County that day, Ron and I decided to hang around until dusk and catch the show. 

As the light fell, more swifts flew in to join the orbiting flock. First there were dozens, then somewhere in the low hundreds. And they kept arriving. I despaired of keeping an accurate count, since birds continued to break away and fly out again. Somewhere around 6:45, the swifts began to fly into the tallest chimney, the leftmost of three. Some appeared to swerve and bounce off at the last second, while others dove in. By now there were easily thousands of swifts in the sky. If they were calling, I couldn’t hear them. 

At about 7, a few birds entered the adjacent, shorter chimney, although the tallest one still had more traffic. Had all the prime locations been taken? The flock tightened: no more flyouts. The light was going fast, but we could still see a handful of swifts in the air at 7:20. 

It occurred to me afterward that the only way to estimate the occupancy of the roost would be to determine the standard rate of swifts flying into the chimney(s) per second and multiply that by elapsed seconds. That’s what Scalf has been doing, using one of those clicker devices. He came up with a total of 6000-plus for the previous Saturday. A week later, a few days after we were there, he counted 6300. Another observer estimated 9000 around midweek. 

In any case, a heck of a lot of Vaux’s swifts, even though one of the brickyard employees told us the numbers were beginning to peter out. Scalf says their persistence at the Marin roost this late in the season is unusual; the birds should be on their way to their Mexican and Central American wintering grounds by now. He says all the known roosts north of San Rafael are empty. 

Unlike their close relative the chimney swift, Vaux’s typically roost in hollow trees rather than man-made structures. There have been some notable exceptions, though: up to 35,000 in a chimney in Portland, where they must have been packed in like sardines, and over 10,000 in the chimney of a school in Healdsburg. Those were fall-migration counts; reported spring roosts tend to be smaller, although a roost in Eugene had 15,000 occupants in early May. 

Vaux’s are classic swifts: small, dark, cigar-shaped. Like most members of their family, they have pamprodactyl feet, with the first and fourth toes freely pivoting forward and backward. This probably helps them keep a grip on vertical roosting surfaces. They build their nests of twigs, held together and to the substrate with saliva. (A relative, the edible-nest swiftlet of Southeast Asia, is the source of the primary ingredient of bird’s-nest soup.) Vaux’s swifts are suspected of mating in flight, although I suspect that would be hard to establish definitively. William S. Vaux, best known as a collector of minerals, was a Philadelphia friend of the 19^th-century naturalist John K. Townsend, who discovered and first described the species. 

The breeding range of the northern (migratory) subspecies of Vaux’s swift extends from southeastern Alaska to central California, as far south as coastal Santa Cruz County and Yosemite National Park. The birds frequent old-growth conifer forests; I’ve seen them among both Douglas-firs and coast redwoods. 

One of several questions raised by the Marin roost is where these birds are coming from. According to the Marin County Breeding Bird Atlas, Vaux’s nests in small numbers in the county, with most observations on Bolinas Ridge. Could there possibly be 6000 breeding individuals in Marin? If not, how large is the catchment area for the roost? And how long have the swifts been using it? 

Another unanswered question: how far down the chimney do the birds go? There’s apparently no data on this point for chimney swifts either.  

It’s probably just as well that Vaux’s swifts haven’t adapted as thoroughly to human structures as chimney swifts have. I remember my first close encounter with swifts, when I was about six or seven my family was living in Little Rock in an older house with a working fireplace. One day a small flock of chimney swifts flew down in and into the living room. Pandemonium ensued. My mother, who had a morbid fear of birds, panicked. The cat went into hunter mode and began bouncing off the walls and furniture in pursuit of the swifts. I believe there was breakage. My father somehow herded the swifts the cat didn’t get into the attic, from which I hope they made their way outside. (I wasn’t allowed in the attic on my own because of the ghost.) And that was the end of the working fireplace.