On Thursday September 18, about 40 people gathered on Wood Street, faces lifted to the sky. For 45 minutes, thousands of migrating chimney swifts spiraled above the old J.W. Hallahan High just north of Logan Circle, wheeling out before cascading back in to form a funnel, and then — nearly all at once — vanishing into a single chimney.

Each year the swifts return in large numbers to roost in favorite chimneys. But that night, the ordinary city evening gave way to something unexpectedly rare and magnificent. You didn’t need to be a birder to feel the pull of it. Swifts are nearly five inches long and look like plump, trim Philly blunts. Field guides describe their calls as “rapid short chips” — but that’s one bird. Multiply it into a widening circle, thousands high over the city at dusk, an

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