Place the chickens, onions, carrots, celery, parsnips, parsley, thyme, dill, garlic, salt, and peppercorns in a 16- to 20-quart stockpot. Add 7 quarts of water and bring to a boil. Lower the heat and simmer uncovered for 4 hours, skimming off any foam that comes to the top. Set aside until cool enough to handle. Strain the entire contents of the pot through a colander and discard the solids.