Preparation: Soak bird's nest in cold water overnight. Drain and rinse. Spread softened nest pieces on plate; pick out prominent pieces of "foreign" matter (e.g. feathers, twigs) with tweezers. Debone chicken breast, remove membrane and muscle fiber, pound meat with cleaver handle to break down tissue, mince chicken until it is pulp. Make medium thick paste with cornstarch and chicken stock. Cooking: Bring rich chicken stock for soup to boil. Immediately add bird's nest; simmer 30 minutes. Mix dry sherry and remaining stock; dribble slowly into minced chicken. Lightly beat egg whites with a fork; fold gently into chicken so they are not completely blended. Add salt to soup. Bring soup back to boil and add chicken mixture slowly so soup does not cool. When soup returns to boil, it is ready to serve. You can hold it at this point on low heat. Pour into serving bowl, garnish with green onions and ham.