What is the primary function of a bird’s crop?

The story behind the answer

The crop is an expandable pouch in a bird’s esophagus used for temporary food storage. It lets birds quickly gather food and hold it to soften before it moves to the proventriculus (for chemical digestion) and the gizzard (for mechanical grinding). It is not the main site of digestion, doesn’t grind food, and doesn’t secrete oil—that last job belongs to the uropygial