Tens of thousands of Vaux's swifts migrating to the U.S. Northwest bunked in a brick chimney in Los Angeles for the night, naturalists who observed them said.

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The birds flew in a spiral Friday night into the chimney of the Chester Building, an abandoned 12-story structure, with two dozen naturalists assembled to watch, the Los Angeles Times reported Saturday.

The birds are about 4 1/2 inches long and cigar-shaped with crescent-shaped wings.

Kimball Garrett, ornithology collections manager at the Natural History Museum of Los Angeles County, told the newspaper the birds are migrating for the summer from Mexico to Oregon, Washington, southeastern British Columbia and Alaska for the breeding season.

He said the abandoned chimney is "a good place for them to sleep."

"They spend all day feeding on nice hatches of insects blown skyward by updrafts along the Los Angeles River and Elysian Park area," he said. "At nightfall, they all come together to roost in the same safe resting area."

The chimney also turned out to be a handy feeding ground for ravens that lay in wait and plucked unlucky Vaux swifts from the migratory stream as they headed for the chimney, the newspaper said.

"It's like watching bears grab salmon out of a river -- where's the 12-gauge shotgun?" Los Angeles Audubon spokeswoman Mary Loquvam said.