It's the thick-walled muscular pouch that grinds food in a bird's lower stomach
[DOC] [TLE] gizzards - Memidex dictionary/thesaurus [PAR] "gizzards" is a plural form of "gizzard": a thick-walled muscular pouch below the ... 
pouch in the lower stomach of many birds and reptiles that grinds food, often with 
... and is used to grind up ingested food before it's transferred to the stomach.