A cuttlebone is not actually a bone, but rather the internal shell of the Cuttlefish, a small, squid-like cephalopod. In the Cuttlefish, the cuttlebone is filled with gasses and helps control the fish's buoyancy in the water. While for years people have harvested and used cuttlebones for various purposes, the most widely recognized use of the cuttlebone is as a supplement and exercise toy for birds.
The cuttlebone is a lightweight, oblong, chalky object that is mainly composed of calcium -- making it an ideal way to supplement a birds's calcium intake.
Cuttlebones have a grainy texture, reminiscent of pumice. This texture can also offer benefits to parrots as it is helpful in wearing down overgrown beaks. It's recommended to always keep a cuttlebone in your bird's cage to promote beak and jaw exercise.
You can usually find a cuttlebone in most grocery or pet supply stores.