When we first got started in rabbits we wanted to raise an endangered breed so that helped us settle on the Silver Fox rabbit. Since then (about 8 years ago), the Silver Fox has moved from the critical to the threatened list. The list also helped us select our duck breed -- Ancona -- which is on the endangered list. The Livestock Conservancy website is very interesting to explore. Here's what their website says about criteria for poultry:
Parameters of Poultry Breeds on the Conservation Priority List
Poultry breeds on the Conservation Priority List generally conform to certain genetic and numerical parameters.
Breeds not meeting all these criteria may be placed in the "Study" category and monitored.