Chickens bred for human consumption (known as broilers) are housed in their thousands in huge sheds, that are not cleaned out for the entire chickens' short, approximately, 6 week lives. These birds are specially designed, by genetic modification, to grow large at a very rapid, unnatural rate.
As they are forced to live in their own urine and faeces, they suffer urine scold on their skins, and their sensitive lungs are damaged due to the intense ammonia smell of their urine. They suffer heart attacks because of their rapid growth and often have broken legs due to the extraordinary weight of their bodies on their tiny bones. Of course with broken legs, they are unable to reach reach food or water. These birds are often cannibalized by other chickens as a more easily reachable food source.
At "harvesting age," they are grabbed by their legs and crammed into crates, then into trucks, by their thousands, without regard for their fragility and sent off to slaughter.
The fully grown, dressed chickens in the supermarkets, are only 6 weeks old.