The Collaborative International Dictionary of English v.0.48:

Chirp \Chirp\, v. i. [imp. & p. p. Chirped; p. pr. & vb. n.
   Chirping.] [Of imitative orgin. Cf. Chirk, Chipper,
   Cheep, Chirm, Chirrup.]
   To make a shop, sharp, cheerful, as of small birds or
   crickets.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Chirp \Chirp\, n.
   A short, sharp note, as of a bird or insect. "The chirp of
   flitting bird." --Bryant.
   [1913 Webster]

WordNet (r) 3.0 (2006):

chirp
    n 1: a sharp sound made by small birds or insects
    v 1: make high-pitched sounds; "the birds were chirping in the
         bushes" [syn: peep, cheep, chirp, chirrup]
    2: sing in modulation [syn: tweedle, chirp]

Moby Thesaurus II by Grady Ward, 1.0:

147 Moby Thesaurus words for "chirp":
   anthem, ballad, bark, bawl, beam, bellow, blare, blat, blubber,
   boom, bray, breathe, buzz, cackle, call, caper, caracole, carol,
   caw, chant, chatter, cheep, chip, chipper, chirk, chirking, chirm,
   chirr, chirrup, chitter, choir, chorus, chuck, clack, clap hands,
   cluck, clutter, cock-a-doodle-doo, coo, creak, crick, croak, cronk,
   croon, crow, cuckoo, dance, delight, descant, do-re-mi, drawl,
   drum, exclaim, exult, flute, frisk, frolic, gabble, gaggle, gambol,
   gasp, glory, glow, gobble, growl, grunt, guggle, hiss, honk, hoo,
   hoot, hum, hymn, intonate, intone, joy, jubilate, keen, laugh,
   lilt, minstrel, mumble, murmur, mutter, pant, peep, pip, pipe,
   psalm, quack, quaver, radiate cheer, rejoice, revel, roar, roll,
   rollick, romp, roulade, rumble, scold, scream, screech, serenade,
   shake, shriek, sibilate, sigh, sing, sing in chorus, skip,
   skip for joy, smile, snap, snarl, snort, sob, sol-fa, solmizate,
   sparkle, squall, squawk, squeal, stridulate, stridulation, thunder,
   tremolo, trill, troll, trumpet, twang, tweedle, tweedledee, tweet,
   twit, twitter, vocalize, wail, warble, whine, whisper, whistle,
   yap, yawp, yell, yelp, yodel