The best information on line breeding specific to chickens I have found has been on the game bird cock fighting forums. I don't approve of the practice, but most of the famous lines have been line bred for decades, some for more than one hundred years.
They have been doing it for thousands of years. They have taught us a lot about poultry, and breeding livestock.
You are right, those sites are a great resource for this kind of information, and poultry in general. Though their goals are different, we can take something from the emphasis and the perspective.
To them, breeding along a line, is about breeding proven performing individuals. The sire and dam is proven, and the offspring are proven. Where we tend to get stuck on color genetics, they are the opposite. They have little regard for color. The emphasis on genotype/phenotype is the opposite of what we find here.
Still a lesson that we can gather from the emphasis is proving the offspring before any real commitment is made to them.
When inheritance is as much or more about the family behind the bird, what we see is not always what we get. I have a cock bird, that I will not use again. He showed promise, but his offspring have showed otherwise. His brother, though he is equal on his own, has been a greater contributor. I intend to use him another season at least.
Good breeders of games, are also very concerned with vigor. Line breeding raises the bar concerning the emphasis on vigor. Maintaining vigor in a line will allow the breeder to go much farther than he/she would have otherwise. Vigor is priority number one. .
Among well bred birds, you will not find any breeds that have vigor equal to the game fowl.
The best game breeders that I have met, do not fight their birds anymore. Many are still at it because they love their birds.
They are also great at conditioning poultry.
