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]