The Collaborative International Dictionary of English v.0.48:

Molt \Molt\, Moult \Moult\, v. i. [imp. & p. p. Molted or
   Moulted; p. pr. & vb. n. Molting or Moulting.] [OE.
   mouten, L. mutare. See Mew to molt, and cf. Mute, v. t.]
   [The prevalent spelling is, perhaps, moult; but as the u
   has not been inserted in the otherwords of this class, as,
   bolt, colt, dolt, etc., it is desirable to complete the
   analogy by the spelling molt.]
   To shed or cast the hair, feathers, skin, horns, or the like,
   as an animal or a bird. --Bacon.
   [1913 Webster] Molt