Animals that are kept for slaughter are bred for fast growth. To reach the desired weight, the animal must have a tendency to eat a lot. When the adult slaughter weight has been reached, the animal is slaughtered. Especially in the case of broiler chickens, this happens extremely fast: after about six to eight weeks. Keeping them alive longer will only cost more food and reduces the profits of the farmer. However, some animals are also needed for producing young time after time.
These are called broiler parent chickens. But to produce many and healthy young, those broiler parents should not be too fat themselves. Just like humans, if these animals become overweight, their reproduction is slowed down.
People used to say, ‘a healthy chicken is not too fat.’  So the broiler parent chickens are given less food than they would like ‘for their own good.’
It would be best if the broiler parents were given a lot of distraction, so they would not only feel a constant craving for food, but also have the opportunity to look for food. The animals are usually kept in dull concrete chicken houses without any distraction. An opening leading to a field, combined with a chicken house covered with a lot of straw in which they can build nests, might provide the necessary distraction.
And when these animals are bred less for fast growth, the well-being of the broiler parents and the animals for slaughter would improve remarkably. Provided that they are kept in a natural environment.