Birds, within the class Aves, are a hugely diverse group of warm-blooded vertebrates. There are over 11,000 living species of birds with a huge variation in their size. The Class Aves are characterised by having feathers, toothless beaked jaws and oviparous reproduction (produce hard-shelled eggs). All bird species have wingsalthough some have evolved over time to be flightless, with wings having vestigial function such as Ratites (i.e., Ostriches) and Sphenisciformes (i.e., penguins). Other island species of birds include the Kakapo, which is the world’s only flightless parrot. All birds that are commonly kept for food production are classified as Poultry. Poultry birds include domestic fowl, turkeys, waterfowl, guinea fowl, quail, pigeons, ratites and game birds (pheasant/partridge) kept in captivity to produce meat, eggs or restocking game. The most common poultry bird is the chicken, who is the ancestor of the red jungle fowl—with domestication thought to have occurred around 8000 years ago in Southeast Asia. Despite thousands of years of captivity only recently has significant research begun into the mechanisms of pain, effective analgesia and minimising suffering in captivity.