The Collaborative International Dictionary of English v.0.48:

Warble \War"ble\, v. i.
   1. To be quavered or modulated; to be uttered melodiously.
      [1913 Webster]

            Such strains ne'er warble in the linnet's throat.
                                                  --Gay.
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   3. To sing in a trilling manner, or with many turns and
      variations. "Birds on the branches warbling." --Milton.
      [1913 Webster]

   3. To sing with sudden changes from chest to head tones; to
      yodel.
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The Collaborative International Dictionary of English v.0.48:

Warble \War"ble\, n. [Cf. Wormil.]
   [1913 Webster]
   1. (Far.)
      (a) A small, hard tumor which is produced on the back of a
          horse by the heat or pressure of the saddle in
          traveling.
      (b) A small tumor produced by the larvae of the gadfly in
          the backs of horses, cattle, etc. Called also
          warblet, warbeetle, warnles.
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   2. (Zool.) See Wormil.
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The Collaborative International Dictionary of English v.0.48:

Warble \War"ble\, v. t. [imp. & p. p. Warbled; p. pr. & vb. n.
   Warbling.] [OE. werbelen, OF. werbler; of Teutonic origin;
   cf. G. wirbeln to turn, to warble, D. wervelen, akin to E.
   whirl. See Whirl.]
   [1913 Webster]
   1. To sing in a trilling, quavering, or vibratory manner; to
      modulate with turns or variations; to trill; as, certain
      birds are remarkable for warbling their songs.
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   2. To utter musically; to modulate; to carol.
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            If she be right invoked in warbled song. --Milton.
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            Warbling sweet the nuptial lay.       --Trumbull.
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   3. To cause to quaver or vibrate. "And touch the warbled
      string." --Milton.
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The Collaborative International Dictionary of English v.0.48:

Warble \War"ble\, n.
   A quavering modulation of the voice; a musical trill; a song.
   [1913 Webster]

         And he, the wondrous child,
         Whose silver warble wild
         Outvalued every pulsing sound.           --Emerson.
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The Collaborative International Dictionary of English v.0.48:

Wormil \Wor"mil\, n. [Cf. 1st Warble.]
   [1913 Webster]
   1. (Zool.) Any botfly larva which burrows in or beneath the
      skin of domestic and wild animals, thus producing sores.
      They belong to various species of Hypoderma and allied
      genera. Domestic cattle are often infested by a large
      species. See Gadfly. Called also warble, and worble.
      [Written also wormal, wormul, and wornil.]
      [1913 Webster]

   2. (Far.) See 1st Warble, 1
      (b) .
          [1913 Webster]

WordNet (r) 3.0 (2006):

warble
    n 1: a lumpy abscess under the hide of domestic mammals caused
         by larvae of a botfly or warble fly
    v 1: sing or play with trills, alternating with the half note
         above or below [syn: warble, trill, quaver]
    2: sing by changing register; sing by yodeling; "The Austrians
       were yodeling in the mountains" [syn: yodel, warble,
       descant]

Moby Thesaurus II by Grady Ward, 1.0:

121 Moby Thesaurus words for "warble":
   air, anthem, ballad, bark, bawl, bellow, blare, blat, blubber,
   boom, bray, breathe, buzz, cackle, call, carol, caw, chant,
   chatter, cheep, chirk, chirp, chirr, chirrup, chitter, choir,
   chorus, chuck, clack, cluck, cock-a-doodle-doo, coo, croak, cronk,
   croon, crow, cuckoo, descant, diapason, do-re-mi, drawl, drum,
   exclaim, flute, gabble, gaggle, gasp, gobble, growl, grunt, guggle,
   hiss, honk, hoo, hoot, hum, hymn, intonate, intone, keen, lay,
   lilt, measure, melodia, minstrel, mumble, murmur, mutter, pant,
   peep, pip, pipe, psalm, quack, quaver, roar, roll, roulade, rumble,
   scold, scream, screech, serenade, shake, shriek, sibilate, sigh,
   sing, sing in chorus, snap, snarl, snort, sob, sol-fa, solmizate,
   squall, squawk, squeal, strain, thunder, tremolo, trill, troll,
   trumpet, tune, twang, tweedle, tweedledee, tweet, twit, twitter,
   vocalize, wail, whine, whisper, whistle, yap, yawp, yell, yelp,
   yodel