Most birds’ digestive systems are like the diagram above, but there are some with slightly, or very, different designs:

These bird groups have no crop:
Ostriches
Emus, Rheas and Cassowaries
Owls

Most birds will produce pellets of undigested fur, claws, fish scales, insect hard parts. But these bird groups are best known for gizzards that collect waste and make it into a pellet which the bird spits out:
Owls
Falcons and Hawks

These bird groups have a special crop which produces milk to feed their young:
Penguins
Pigeons
Flamingos

This bird group has a bladder, (unlike any other bird):
Ostrich