expandable section at the low end of the esophagus where food can be stored for a time after it is swallowed before it is pushed by contracting muscles into the upper stomach (proventriculus); enables birds to ingest more food than they need immediately; no digestion occurs in the crop; found in diurnal raptors, grouse, some songbirds (especially seed-eaters), pigeons, and doves; falconers have referred to the neck-stretching, gyrating, downward-pushing action of moving the food out of the crop to the stomach as “putting over”; “crop milk” produced by pigeons and doves (but not raptors) consists of nutrient-rich cells from the lining of the crop that are regurgitated to feed young