The long list of land-birds groups is headed by the order Columbiformes, which is dominated by the largely uniform family of pigeons and doves. These are tree- or ground-inhabiting birds that feed primarily on seeds and fruit. Nearly 300 species strong, members of the pigeon and dove family are found worldwide except in the highest latitudes. Pigeons and doves feed their young with a milky substance produced by the adult birds -- a trait that is unique to the family. Ornithologists believe that pigeons are related to the extinct dodos and solitaires of the islands of the Indian Ocean. The order Columbiformes also includes the sandgrouse family. The open-country birds of Africa and southern Eurasia share some of the characteristics of pigeons and some of the characteristics of the Charadriiformes. As a result taxonomists sometimes assign these birds to the order Columbiformes and sometimes to Charadriiformes.