Cosette is referred to as a "lark" in "Fantine." It is a nickname she is given by people in the town because she is so small and up before the sun. Sadly, "the lark never sang."
In "Cosette," when Jean Valjean rescues Cosette and the two are on the run together, they referred to by the author (or narrator) as "the owl and the wren." A sectional title in the unabridged version of the novel is "A nest for the owl and the wren." The owl, of course, is Valjean. The reference pertains both to his age and his wisdom or resourcefulness. "Wren" then is used to show that Cosette is still small and birdlike, but she is no longer under the same identity she had while working at the Thenardier's inn. Perhaps this bird will both sing and eventually fly.