I think a lot of the chicken sold in the UK comes from unethical, overcrowded chicken farms that develop fast growing chicken breeds, otherwise known as 'Frankenchickens'.
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The reason that I think these farms are unethical is because I believe the chickens live a life of suffering as they have been modified to eat unnatural quantities of food to fuel their fast growth. This can lead to many problems, including their inability to walk and often laying in their own waste. The thing that angers me most is that I think most people have no awareness of this issue and continue to buy cheap chicken that had been raised in what I think is a disgraceful way.