18. Systems of keeping hens can be classified as intensive or extensive. Intensive housing systems include cages, percheries, aviaries, deep litter and covered yards. Free range systems are extensive.
19. Percheries and aviaries have been in common use for about 15 years. They are multi-level systems which encourage birds to use the three dimensions of a building rather than just the floor level. Most systems provide feed, water and nesting facilities, as well as perches or platforms, at several levels. Most, but not all, incorporate littered areas.
20. Deep litter systems are also still in use. They generally have about 50% of the floor area as litter and the remainder as a raised perforated floor over a manure collection area.
21. Free range production can consist of any of the above housing systems, except cages, but with the essential proviso that hens should have continuous day time access to land mainly covered with vegetation.
22. Some functions of most of the systems can be mechanically controlled, and in modern larger units, feed and water provision and egg collection are often automated. Within each system there may be considerable variation in design detail. Systems seen during the study are listed at Appendix C.