WASHINGTON, May 18 -- Smithsonian's National Zoo and Conservation Biology issued the following news release: Carnivore keepers at the Smithsonian's National Zoo and Conservation Biology Institute (NZCBI) in Front Royal, Virginia, welcomed a litter of endangered black-footed ferrets this week. One-year-old female Aristides gave birth to six kits May 11. Animal care staff are closely monitoring the ferrets' behaviors through the Black-Footed Ferret Cam, a temporary live webcam on the NZCBI websit  more