Stress comes in many forms such as heat stress, wind stress, cold stress, and a big one, over handling stress. A more docile bird that doesn't stress as easy comes into play here. Chicks that are more adaptable to change and settle quicker are not going to be stressed as much. Even though environment plays a large portion, the genetics are a big factor. There is a difference in feed efficiency among birds.
Feed conversion is a heritable trait and can be selected for to a degree.
The size of the bird as well as the temperament can affect feed consumption. Common sense tells you a bird that is constantly walking or running the fence is going to require more feed because he is using more energy. Also, the larger the bird, the more feed he requires. It is fine to invest more feed in a bird if that bird produces more product. Basically all of our products from the ostrich come from one area, the body.