Some bird species have an expanded, thin-walled offshoot or pouch in the oesophagus, which is called the crop. The crop is being used to store food before it is being transferred through the oesophagus into the stomach. Corvids don’t have a genuine crop.
However, all bird species, including corvids, have two parts to their stomach. The first part of the stomach is the proventriculus or glandular stomach, where digestive enzymes are secreted to initiate the process of digestion. The second part of a bird’s stomach is the gizzard or muscular stomach.
The gizzard is very thick and muscular in some species, in particular in those where cereals are an important part of the diet. The gizzard is being used for grinding food, typically with grit. Other species, in particular those which favour soft or easily digested foods, have usually a very small and thin-walled gizzard. The gizzard is also the part of stomach, where ‘pellets’ are being produced, which consist of the remaining non-digestible food materials like bones, fur or feathers. This may include directly or voluntarily ingested grit and indirectly via the consumption of seed eating birds.
All of our native corvid species seem to use, at least to some degree, grit to aid digestion, which is particular evident in jackdaws, carrion crow and ravens. It is assumed that this helps to break down the ingested food. In jackdaws and carrion crows there is also evidence that calcareous grit is an essential source of calcium in particular needed for eggshell formation.1
Depending on seasonal and species related diet variations and nutritional needs, corvids will choose, how much and what kind of grit they require to digest their food and also to supplement their mineral needs. Therefore it seems a good idea to provide a variety of different minerals like sand, grit and other supplements like cuttlefish bone or cleaned snail shells.