Not all species of birds possess a gall bladder, in particular, most species of parrots, pigeons and ostriches.

The dove or pigeon, from ancient times, was considered special because of this anatomical difference, as the "gall" was considered the seat of bitterness in an individual. Hence the dove or pigeon, being without "gall", lacked some of the more unpleasant aspects of personality, shared by man other beasties.

Apparently there are Biblical tales about the dove bursting its gall bladder in grief when Nohah released it from the Ark, as an explanation for the absence.