Place the chicken in a 6-quart pot. Add all the ingredients through the parsnip, plus the parsley sprigs, lemon slice, peppercorns, and 1 tsp salt. Add the chicken stock and enough water to cover the chicken by 2 inches. Heat to boiling, skimming any residue as it gathers. Reduce the heat and simmer, partially covered, 1 hour.
Remove the chicken from the stock and set the stovk aside. Allow the chicken to cool enough to handle, then carefully remove the meat from the bones and cut it into bite-size pieces. Cover and reserve. Return the bones and skin to the stock. Returen it to the boil; reduce the heat and simmer, uncovered, 45 minutes. Strain the stock into another pot, pressing the vegetables gently with the back of a spoon to release their juices. Meanwhile, saute the sausage pieces in a lightly oiled heavy skillet until well browned, about 8 minutes. Drain on paper towels. Add the sausages to the stock. Heat to boiling; reduce the heat. Simmer, uncovered, skimming any fat that rises to the surface, for 20 minutes. Add the pasta shells; cook 10 minutes longer. Add the zucchini and the reserved chicken; cook 5 minutes. Add salt and pepper to taste, and sprinkle with the chopped parsley.