The Livestock Conservancy explains the definition of a heritage breed of chicken. And even though they list Delaware, New Hampshire, and Plymouth Rock as heritage breeds, these breeds do not meet The Livestock Conservancy's own definition because these three breeds mature faster than 16 weeks, according to their chart. By the way, that seems very arbitrary to me. http://www.livestockconservancy.org/index.php/heritage/internal/chicken-chart http://www.livestockconservancy.org/index.php/heritage/internal/heritage-chicken Heritage Chicken must adhere to all the following: APA Standard Breed Heritage Chicken must be from parent and grandparent stock of breeds recognized by the American Poultry Association (APA) prior to the mid-20th century; whose genetic line can be traced back multiple generations; and with traits that meet the APA Standard of Perfection guidelines for the breed. Heritage Chicken must be produced and sired by an APA Standard breed. Heritage eggs must be laid by an APA Standard breed. Naturally mating Heritage Chicken must be reproduced and genetically maintained through natural mating. Chickens marketed as Heritage must be the result of naturally mating pairs of both grandparent and parent stock. Long, productive outdoor lifespan Heritage Chicken must have the genetic ability to live a long, vigorous life and thrive in the rigors of pasture-based, outdoor production systems. Breeding hens should be productive for 5-7 years and roosters for 3-5 years. Slow growth rate Heritage Chicken must have a moderate to slow rate of growth, reaching appropriate market weight for the breed in no less than 16 weeks. This gives the chicken time to develop strong skeletal structure and healthy organs prior to building muscle mass.