The proventriculus is part of the digestive system of birds. An analogous organ exists in invertebrates and insects.
The proventriculus is a standard part of avian anatomy. It is generally a glandular part of the stomach that may store and/or commence digestion of food before it progresses to the gizzard.
The Encarta (2007) holds that the proventriculus is:
The first part of a bird's stomach, where digestive enzymes are mixed with food before it goes to the gizzard. It is analogous to the gizzard in insects and crustaceans.
Thomas Caceci (College of Veterinary Medicine of VirginiaTech) discusses the proventriculus of the avian stomach and opines that:
The proventriculus is the glandular portion of the avian compound stomach, and a rather peculiar organ it is. There's nothing like it in mammals.
Chicken proventriculus is eaten as street food in the Philippines. This dish is called proben.