In a stock pot, or other large pot, bring 2 gallons of water to a boil. Reduce the heat to a slow simmer. Preheat the oven to 400°F.
Place the reserved bones and necks in a roasting pan. Place in the oven and roast until the bones are a rich golden brown, about 30 minutes. As soon as the carcasses are browned, place them in the pot. Simmer until the stock has reduced by half and has a rich poultry flavor, about 2 to 3 hours. Strain and refrigerate until ready to use.