As the sun disappears behind the hills in Northwest Portland, hundreds of people lay sprawled upon blankets beside Chapman Elementary School.

Everyone's attention is split between the kids sledding down the well-worn, grassy slopes on cardboard — and the sky.

Swift watch is a beloved Portland tradition. | Photo via PDXtoday

They're all waiting to see the Vaux's swifts — little birds that look like cigars with wings — which return to sleep in the school's chimney late August through September. But before they rest, they swoop and dive, munching on flying insects while putting on a show with their aerial acrobatics.

This particular group is thought to be the largest roost of the species in the world. They've been making the same layover on their migration route to Central America and Venezuela since the late 1980s.

Volunteers with Bird Alliance of Oregon are on-site every weeknight to count the swifts and share interesting facts — like how Vaux's swifts don't perch, but instead cling to vertical surfaces like trees or brick structures.

Peregrine falcons and other birds of prey often make an appearance too, licking their beak for what's an almost guaranteed meal.

As many as 7,000 swifts roost in the chimney. | Gif via PDXtoday

The anticipation builds with each speculative swirl of swifts, until by some unknown signal, they begin to funnel down the hole by the thousands, eliciting cheers from their spectators.

If you'd like to see the feathered phenomenon for yourself, there are several things you should know:

Arrive about an hour before sunset and bring something comfy to sit on.
Parking in the area is extremely limited around this time, so be prepared to walk or take public transportation on TriMet buses #77 or #15 to Northwest Vaughn Street + 26th Avenue. Bike racks are also available.
Dogs are permitted on a leash.
If you pack it in, pack it out.