In regard to their

internal parts birds differ from other animals
and from one another. Some birds, for instance, have a crop in front
of the stomach, as the barn-door cock, the cushat, the pigeon, and the
partridge; and the crop consists of a large hollow skin, into which
the food first enters and where it lies ingested. Just where the
crop leaves the oesophagus it is somewhat narrow; by and by it
broadens out, but where it communicates with the stomach it narrows
down again. The stomach (or gizzard) in most birds is fleshy and hard,
and inside is a strong skin which comes away from the fleshy part.
Other birds have no crop, but instead of it an oesophagus wide and
roomy, either all the way or in the part leading to the stomach, as
with the daw, the raven, and the carrion-crow.