In a North American context the terms market hunter and commercial hunter are predominantly used to refer to hunters of the 19th and early 20th century who sold or traded the flesh, bones, skins and feathers of slain animals as a source of income. These hunters focused on species which gathered in large numbers for breeding, feeding, or migration and were organized into factory-like groups that would systematically depopulate an area of any valuable wildlife over a short period of time. The animals which were hunted included bison, deer, ducks and other waterfowl, geese, pigeons and many other birds, seals and walruses, fish, river mussels, and clams.[23]