In my experience, the processor is actually the one who owns the chickens (broilers). The farmer is essentially a baby-sitter for 6-7 weeks. The processor also owns the feed and the farmer gets paid on a feed-to-poultry conversion efficiency.

Variables like temperatures, lighting, ventilation rates, chicken house construction, and access/ density have well-established bounds to maximize pounds of poultry. The farmers have leeway to deviate from the recommendations, but they take on all of the risk in doing so. From the outside, it looks like a pretty oppressive relationship.