Their dark skin screens out sunlight and tends to deprive them of vitamin D3 which forms in skin exposed to sunlight. Absence of vitamin D3 produces rickets. There are exceptions, however. Their diet, which includes fish-liver oils containing large amounts of vitamin D3, prevents rickets. This has been demonstrated in geese.
Blue snow geese live in one region of the Arctic, and white snow geese live in another.
Eggs from each colony were hatched in an incubator. The young geese later showed a mating preference for geese having the color of their foster parents. In another experiment, the foster parents were painted pink. Again there was a mating preference for the color the young geese saw as they were growing up, even though that color was artificial. Prejudices based on physical appearances have caused wars, genocide, forced segregation, and voluntary isolation.