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Chickens are the most ubiquitous of all livestock species, and are to be found more or less everywhere inhabited by people. Because of their rapid turnover and efficient conversion of feed into protein they provide the first response to growing demand because efficient production units can be set up very quickly. Between 1960 and 2010 the global stocks of chickens increased 5 times and average carcass weights more than doubled. This gave rise to a roughly 12-fold increase in chicken meat over the 50-year period. Most of this growth has been in intensive units specialized in meat or egg production, rather than in backyard systems that tend to raise dual-purpose birds for home consumption and local sale.