Many Heritage Breeds are endangered, some are critically so. With limited populations remaining, restored breeds may be something other than what the original breed was.
When restoring a nearly lost breed, some breeders will cross breed, to introduce more genetic diversity. Done poorly, this can completely destroy the original defining characteristics of a breed.
Other breeders will preserve ALL surviving offspring to use as future breeders, rather than culling to enhance breed traits. Non-conforming animals will be preserved, and sometimes even animals with serious defects will be preserved as breeding stock. This also has the potential to destroy a breed - because the very thing that defines it may be corrupted.
A delicate balance must be preserved in order to rescue a breed. In the first generations, ALL serious defects must be culled. In successive generations, more aggressive culling is required to refine the breed traits and restore them to a close approximation of the original.
The most common traits to be overlooked or ignored completely, are those that are NOT visible in appearance. Traits such as productivity, disease hardiness, thriftiness, parenting ability, and disposition are frequently ignored, yet these traits are very often the core reason why the breed existed initially, and the most desired traits by potential buyers.
Commercial production lines have entered the arena of "preserving" poultry breeds, which has resulted in a large population of birds which bear the name of a breed, but which lack any but the most obvious traits of the breed.
If you choose to raise heritage animals with the goal of aiding in preserving the breed, please breed carefully. Cull for defects that are not in compliance with the breed standard, and pay attention to less visible traits. This kind of excellence makes the entire breed better, and it pays off when selling your livestock.
If preserving breeds is worth doing, it is worth doing right.