Digestion a thin-walled expanded portion of the alimentary tract used for the storage of food prior to digestion. In some birds it is an expanded, muscular pouch near the gullet or throat. In adult doves and pigeons, the crop can produce crop milk to feed newly hatched birds. Certain insects may have a crop or enlarged esophagus. Herbivores have evolved cecums (or an abomasum in the case of ruminants). Ruminants have a fore-stomach with four chambers. These are the rumen, reticulum, omasum, and abomasum. In the first two chambers, the rumen and the reticulum, the food is mixed with saliva and
Birds of the Danube Delta (29); Mongolian (33); Chinese (2). The transpalearctic type contains 43 species. For Romania's latitude – delta included – the north-originating species (Arctic and Siberian) appear as winter-and passage birds, while the south-originating species (European, Mediterranean and Mongolian) are met with as sedentary and summer species. Further comments upon the manner birds migrate - namely the causes, which start on migration - and upon the way they orient themselves during migration, resulted in the hypothesis of the ""inner time"" and ""inner space"", which, along with their known senses, allow the migratory birds to perfect the migratory act in time and space.