Autumn’s arrival has brought the return of migrating Vaux’s swifts to Rio Lindo Adventist Academy in Healdsburg, where the tiny birds have been roosting by the thousands each night over the past 5½ weeks, putting on evening shows for those lucky enough to bear witness.

The birds have sought refuge at the Fitch Mountain campus each fall since 1989, funneling at dusk into a huge brick chimney where they pack themselves tightly to stay warm as the nights grow chilly.

An estimated 21,000 were counted one night last week at the private high school. Their evening ritual typically starts with a feeding frenzy that fills the sky before the birds form a whirling mass and head into the chimney.

The boarding school is one of several dozen monitored roosting sites in the western United States where the swifts are known to stop during their annual southward journey from nesting sites as far north as Alaska and Canada’s Yukon Territory to wintering grounds in Mexico and points south.

Their numbers vary greatly, with a high count above 35,000 during several evenings in 2015. In 2005, the highest count was 300, according to information on the school’s website.

Last year’s peak was 29,000, according to records kept by Vaux’s Happening, a Western states research project.

The project coordinator, Larry Schwitters, said the numbers overall are high this year around the region, despite what’s believed to be an ongoing, slight annual drop in total population. He said it appears wildfires and windblown smoke have shifted the migration westward in some areas, raising numbers at some monitored sites while diminishing others.

He also said it appears there is a new site in Stewart’s Point, in far northwestern Sonoma County, where several thousand swifts have reportedly roosted on recent nights.

At Rio Lindo, where the swifts began arriving in mid-August, the numbers climbed for several weeks, reaching about 21,000 Sept. 13 and dropping since then. At last check, Sept. 18, the birds were estimated at 7,000.

You can reach Staff Writer Mary Callahan at 707-521-5249 or mary.callahan@pressdemocrat.com. On Twitter @MaryCallahanB.