Crows at dusk

By design, birds merge
in the sudden cause of flocks.
Forms lock and unlock as one
breath, like a M.C. Escher sketch.
Dozens circle above rush hour,
over engines, brakes, horns;
caws etch the air, echo off
the rough rouge of brick walls.
Crusts of old snow lie weeping
on cold sidewalks this evening,
while brittle lace still decorates
the many bare arms of bark.
One by one, crows fold themselves,
arranged into the sculptures of trees,
heavy as wrought-iron leaves.