Swan songThe sound supposedly produced by a dying swan (genus Cygnus). The origin of the expression is uncertain but presumably it refers to the loud-voiced Whooper Swan C. cygnus or Bewick's Swan C. columbianus, rather than the Mute Swan C. olor. Possibly exhalation of air through the trachea of a shot swan as it falls from the sky can activate the vocal cords and produce an unusual and melodious sound at times.
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From Peter Weaver's Birdwatcher's Dictionary
Copyright © 1981 by Peter Weaver