It would take a decade of indiscriminate inbreeding to effect the birds vigor. Even then it might not have much effect. Closed flocks that have been properly line bred for 30 years still have vigor. That's a big problem with too closely breeding and not going back to aunts and uncles; eventually you start to lose vigor in the flock. If all other signs are not seen at least poor hatch results will be a clue it's time to add new blood to improve vigor of flock.
The other problem with too close of breeding (brothers and sisters over generations) is faults of the breed are more readily going to show up. As much as your improving by using the positive attributes of close relatives there will be recessive negative attributes that can pair up to express in offspring too. Some things take several locations of paired genes to express and you'd not likely see the result unless too closely inbreeding over generations. It's like getting hit by lightning, not much of a chance unless you like to watch the storm from aluminum boats. You really have to go out of your way to increase the odds. Line breeding in itself when properly managed can do amazing things with livestock. Father to daughters mating and the like is how desired attributes are improved. No breed of livestock came to be created without it.