Humans and animals alike have instincts that they are born with, regardless of where they are raised. In addition to the physical suffering—muscle pain, skeletal deformities, respiratory illnesses, and other chronic ailments—caused by intensive confinement, being prevented from performing the most basic instinctual behaviors causes tremendous mental anguish. Even animals caged since birth want to move around, groom themselves, stretch their limbs or wings, and exercise. Studies at Bristol University in England revealed that when factory-farmed chickens who grew up crammed into wire cages were freed, they naturally began to dust-bathe and build nests for their eggs. Herd animals and flock animals become distressed when they are made to live in isolation or when they are put in groups too large for them to be able to recognize other members.
In addition, all confined animals suffer from intense boredom—some so severely that it can lead to self-mutilation or other self-destructive behavior, which farmers “solve” by pulling teeth and chopping off beaks and tails.