Merry as a Cricket
or as a Lark, or as a Grig. The French say, “Fou (or Folle) comme le branlegai, ” and more commonly “Gai comme un pinson” (a chaffinch). “Branlegai” is a dance, but the word is not in use now.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894