Most meat birds are hybrids, meaning that they are cross bred. Most common that I am aware of is white rock X cornish. I could be wrong, but logic would dictate a cornish cock is mated with white rock hens, the reason being that rock hens are better layers, but it could easily be done the other way around, or RIR hens, etc could be used. The reason for using white birds is ease of plucking. When crossing breeds like this, you get what is known as "hybrid vigor", which is basically the best of both breeds, thus producing a fast growing, heavy bird. Mating two birds of the same hybrid vigor cross never reproduces offspring with "hybrid vigor", so to reproduce the big meat birds, you need to make the cross that produced them in the first place. Clear as mud? I hope I`m explaining this so most can grasp it. One more thing that inhibits mating like meat birds, or keeping them to laying age, is that rapid growth produces bone problems and many birds, when grown to laying age, merely have too many physical problems to make it feasable.........Pop