[1] : The Collaborative International Dictionary of English v.0.48

watcher \watch"er\ -~er, n.
   one who watches; one who sits up or continues; a diligent
   observer; specifically, one who attends upon the sick during
   the night.
   1913 webster

[2] : WordNet (r) 2.0

watcher
     n 1: a close observer; someone who looks at something such as an
          exhibition of some kind; "the spectators applauded the
          performance"; "television viewers"; "sky watchers
          discovered a new star" syn: spectator, witness, viewer,
           looker
     2: a guard who keeps watch syn: watchman, security guard
     3: a person who keeps a devotional vigil by a sick bed or by a
        dead body
see also:
spectator witness viewer looker watchman security guard 

[3] : Moby Thesaurus II by Grady Ward, 1.0

52 moby thesaurus words for "watcher":
   argus, charley, tv-viewer, airplane spotter, beholder,
   bird-watcher, bystander, drugstore cowboy, eyewitness,
   fire patrolman, fire warden, fireguard, forward observer, gaper,
   gazer, gazer-on, girl-watcher, goggler, kibitzer, looker,
   looker-on, lookout, lookout man, night watchman, observer, ogler,
   onlooker, patrol, patroller, patrolman, perceiver, percipient,
   picket, roundsman, scout, seer, sentinel, sentry,
   sidewalk superintendent, spectator, spectatress, spectatrix,
   spotter, televiewer, television-viewer, vedette, video-gazer,
   viewer, watch, watchkeeper, watchman, witness

Watcher >> W Words
Watcher, definition of term: Watcher
watcher_pag1.html

Bird Watcher >> B Words
Bird Watcher, definition of term: Bird Watcher
bird+watcher_pag1.html