Rub ducks liberally with olive oil, salt and pepper. Place in a heavy roasting pan with onion, celery and carrots. Place uncovered in a preheated 400 degree oven for 45 minutes, turning contents occasionally until well browned. Add chicken broth, cover, reduce temperature to 375 and roast until meat pulls easily off body, about 2 hours more. Allow to cool and pull meat off carcass. Keep meat warm.