The proventriculus is part of the digestive system of birds, invertebrate

Invertebrate

An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...

s and insect

Insect

Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...

s.

Birds

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

Gizzard

The gizzard, also referred to as the ventriculus, gastric mill, and gigerium, is an organ found in the digestive tract of some animals, including birds, reptiles, earthworms and some fish. This specialized stomach constructed of thick, muscular walls is used for grinding up food; often rocks are...

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The Encarta

Encarta

Microsoft Encarta was a digital multimedia encyclopedia published by Microsoft Corporation from 1993 to 2009. , the complete English version, Encarta Premium, consisted of more than 62,000 articles, numerous photos and illustrations, music clips, videos, interactive contents, timelines, maps and...

 (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.

Dr. Thomas Caceci (undated) discusses the proventriculus of the avian stomach

Stomach

The stomach is a muscular, hollow, dilated part of the alimentary canal which functions as an important organ of the digestive tract in some animals, including vertebrates, echinoderms, insects , and molluscs. It is involved in the second phase of digestion, following mastication .The stomach is...

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.