Cuttlefish bone for the use to supply calcium for egg layers such as birds and invertebrates/reptiles with shells. In pet birds Cuttlefish bone serves as a source of calcium, which is lacking in seed. It also provides your bird with a hard surface to gnaw on, which should help to prevent their bill from becoming overgrown.
Cuttlefish bone is a long-standing method of providing extra calcium to tortoises, and especially for turtles, as it floats readily in water. The main chemical constituents of cuttlebone are calcium carbonate, sodium chloride, calcium phosphate, magnesium salts and an abundance of micro-trace elements. Despite its high gross calcium content, cuttlefish bone is relatively poorly absorbed, and as such should not be relied upon as the sole source of calcium supplementation.