The Collaborative International Dictionary of English v.0.48:

Tattler \Tat"tler\, n.
   1. One who tattles; an idle talker; one who tells tales.
      --Jer. Taylor.
      [1913 Webster]

   2. (Zool.) Any one of several species of large, long-legged
      sandpipers belonging to the genus Totanus.
      [1913 Webster]

   Note: The common American species are the greater tattler, or
         telltale (Totanus melanoleucus), the smaller tattler,
         or lesser yellowlegs (Totanus flavipes), the solitary
         tattler (Totanus solitarius), and the semipalmated
         tattler, or willet. The first two are called also
         telltale, telltale spine, telltale tattler,
         yellowlegs, yellowshanks, and yelper.
         [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Telltale \Tell"tale`\, n.
   1. One who officiously communicates information of the
      private concerns of others; one who tells that which
      prudence should suppress.
      [1913 Webster]

   2. (Mus.) A movable piece of ivory, lead, or other material,
      connected with the bellows of an organ, that gives notice,
      by its position, when the wind is exhausted.
      [1913 Webster]

   3. (Naut.)
      (a) A mechanical attachment to the steering wheel, which,
          in the absence of a tiller, shows the position of the
          helm.
      (b) A compass in the cabin of a vessel, usually placed
          where the captain can see it at all hours, and thus
          inform himself of the vessel's course.
          [1913 Webster]

   4. (Mach.) A machine or contrivance for indicating or
      recording something, particularly for keeping a check upon
      employees, as factory hands, watchmen, drivers, check
      takers, and the like, by revealing to their employers what
      they have done or omitted.
      [1913 Webster]

   5. (Zool.) The tattler. See Tattler.
      [1913 Webster]

   6. A thing that serves to disclose something or give
      information; a hint or indication.

            It supplies many useful links and telltales.
                                                  --Saintsbury.
      [Webster 1913 Suppl.]

   7. (Railroads) An arrangement consisting of long strips, as
      of rope, wire, or leather, hanging from a bar over
      railroad tracks, in such a position as to warn freight
      brakemen of their approach to a low overhead bridge.
      [Webster 1913 Suppl.]
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Telltale \Tell"tale`\, a.
   Telling tales; babbling. "The telltale heart." --Poe.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Yellowlegs \Yel"low*legs`\, n. (Zool.)
   Any one of several species of long-legged sandpipers of the
   genus Totanus, in which the legs are bright yellow; --
   called also stone snipe, tattler, telltale,
   yellowshanks; and yellowshins. See Tattler, 2.
   [1913 Webster]

WordNet (r) 3.0 (2006):

telltale
    adj 1: disclosing unintentionally; "a telling smile"; "a
           telltale panel of lights"; "a telltale patch of oil on
           the water marked where the boat went down" [syn:
           revealing, telling, telltale(a)]
    n 1: someone who gossips indiscreetly [syn: tattletale,
         tattler, taleteller, talebearer, telltale,
         blabbermouth]

Moby Thesaurus II by Grady Ward, 1.0:

98 Moby Thesaurus words for "telltale":
   a tale-bearing animal, babbling, betrayal, betrayer, blab, blabber,
   blabberer, blabbering, blabbermouth, blabbing, broad hint,
   busybody, catchword, clack, clue, communication leak, cue,
   cue word, dead giveaway, delator, divulgation, divulgement,
   divulgence, divulging, evidence, evulgation, fink, gentle hint,
   gesture, giveaway, glimmer, glimmering, gossip, gossip columnist,
   gossiper, gossipmonger, hint, hot lead, implication, index,
   indication, indiscretion, informer, inkling, innuendo, insinuation,
   intimation, key, key word, kick, lead, leak, letting out, look,
   narc, newsmonger, newspaperman, nod, notion, nudge, obvious clue,
   peacher, prompt, quidnunc, reporter, rumormonger, scandalmonger,
   scent, sign, signal, snitch, snitcher, spoor, spy, squealer,
   stool pigeon, stoolie, suggestion, suspicion, symptom, tabby,
   talebearer, taleteller, tattler, tattletale, telltale sign,
   tip-off, tittle-tattler, trace, track, unwitting disclosure,
   vestige, whiff, whisper, whistle-blower, wind, wink, yenta