With darkening fall skies, the birds become restless, gathering in swirls until the horde becomes a churning 30,000-bird whirlpool heading...

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With darkening fall skies, the birds become restless, gathering in swirls until the horde becomes a churning 30,000-bird whirlpool heading straight for an old chimney at Chapman Elementary School in Portland, one of the most spectacular events in all of birddom.

We have a smaller, but no less impressive roost at Monroe Elementary School in Monroe, where an estimated 4,000 Vaux’s swifts (pronounced “Vox’s”) spiral down the decommissioned chimney all at once, like a cartoon cyclone, or a giant vacuum cleaner sucking them in.

This is a fall stopover on the swifts’ way to Mexico and Central America. The birds arrive here around the end of August and are completely gone by October. You too could host hundreds of birds in your home, as one house in Enumclaw did several years ago; the birds actually entered their living room.

Vaux’s swifts (Chaetura vauxi) are a 4-inch long, cigar-shaped bird that can fly at a smokin’ 100 mph, devouring thousands of mosquitoes and other flying insects in their path. They eat, mate and drink all while flying because, as their family name Apodidae (no feet) implies, they cannot readily stand, walk or hop — they cling. They have feet, they are just tiny.

Vaux’s swift is our Western version of the Eastern chimney swift. In addition to chimneys, they roost in large hollow old-growth trees, which are becoming very rare, as are the old brick industrial chimneys at those elementary schools.

Because the Vaux’s swift is losing habitat, it is listed as a Washington Department of Fish and Wildlife’s “priority species.” It’s also a candidate for Washington’s threatened or endangered species list.

You should not allow birds and mammals to enter your home’s chimney. Only swifts and bats, and sometimes raccoons, can free themselves from a chimney flue. Most get trapped and perish horribly. Always install an appropriately-sized and professionally designed chimney cap to keep critters out. Then, go see the Monroe roost. It will blow you away.

Patricia Thompson is a wildlife biologist with the Washington Department of Fish and Wildlife. She can be reached at http://wdfw.wa.gov.