Poultry includes all birds domesticated by man. They are kept for their eggs and for their meat. They include chickens, turkeys, geese and ducks. Chickens are mainly used throughout the world for egg production. Methods for keeping poultry range from free-range systems, where the birds can roam as they will but at night they are kept in barns for their own protection, through semi-intensive systems where they are kept in barns and have perches, litter and some freedom of movement, to intensive systems where they are kept in cages.