The sun has barely relinquished its hold on the sky. Twilight transforms lush green aspens and firs into foreboding black shapes that whisper threats with each breeze, and scratch against the night. A Swainson's thrush perches on an ancient snag and pumps his melodious "reeger-Reeger-REEGER!" into the gloaming until the night quiets him.The song is meager antidote to the paranoia that lions! and tigers! and bears, oh, my! are descending through the blackness to dine on a human listener rarely found holding a mountain vigil at 9:30 on a summer's night.And then another sound seeps into the consciousness, low-pitched and ambiguous, at first imaginary, then real: "Booop! Booop! Boo-doop, Booop!" The sound makes the heart sing and relaxes the electrified hairs on the back of the neck. The singer is a flammulated owl.