Gizzard stone in the context of "Ballast"

The dodo (Raphus cucullatus) is an extinct flightless bird that was endemic to Mauritius, an island east of Madagascar in the Indian Ocean. The dodo's closest relative was the also-extinct and flightless Rodrigues solitaire. The two formed the subtribe Raphina, a clade of extinct flightless birds that are a part of the group that includes pigeons and doves (the family Columbidae). The closest living relative of the dodo is the Nicobar pigeon. A white dodo was once thought to have existed on the nearby island of Réunion, but it is now believed that this assumption was merely confusion based on the also-extinct Réunion ibis and paintings of white dodos.

Subfossil remains show the dodo measured about 62.6–75 centimetres (2.05–2.46 ft) in height and may have weighed 10.6–17.5 kg (23–39 lb) in the wild. The dodo's appearance in life is evidenced only by drawings, paintings, and written accounts from the 17th century. Since these portraits vary considerably, and since only some of the illustrations are known to have been drawn from live specimens, the dodos' exact appearance in life remains unresolved, and little is known about its behaviour. It has been depicted with brownish-grey plumage, yellow feet, a tuft of tail feathers, a grey, naked head, and a black, yellow, and green beak. It used gizzard stones to help digest its food, which is thought to have included fruits, and its main habitat is believed to have been the woods in the drier coastal areas of Mauritius. One account states its clutch consisted of a single egg. It is presumed that the dodo became flightless because of the ready availability of abundant food sources and a relative absence of predators on Mauritius. Though the dodo has historically been portrayed as being fat and clumsy, it is now thought to have been well-adapted for its ecosystem.

The Rodrigues solitaire (Pezophaps solitaria) is an extinct flightless bird that was endemic to the island of Rodrigues, east of Madagascar in the Indian Ocean. A member of the family of pigeons and doves, it was most closely related to the also extinct dodo of the nearby island Mauritius, the two forming the subfamily Raphinae. The Nicobar pigeon is their closest living genetic relative.

Rodrigues solitaires grew to the size of swans, and demonstrated pronounced sexual dimorphism. Males were much larger than females and measured up to 75.7–90 centimetres (30–35 inches) in height and 28 kilograms (62 pounds) in weight, contrasting with 63.8–70 centimetres (25–28 in) and 17 kilograms (37 lb) for females. Its plumage was grey and brown; the female was paler than the male. It had a black band at the base of its slightly hooked beak, and its neck and legs were long. Gizzard stones helped digest its food, which included fruit and seeds. Both sexes were highly territorial, with large bony knobs on their wings that were used in combat, probably due to competition for the limited resources on the island. The Rodrigues solitaire laid a single egg that was incubated in turn by both sexes.