The Smithsonian’s National Zoo has announced the birth of two Andean bear cubs, born to 4-year-old Billie Jean back in mid-January. The twins are believed to have been over a 24-hour period Jan. 14-15, but momma bear and her offspring have been secluded in a private indoor habitat, so all zookeepers know for sure at this point is what they can see and hear via closed-circuit camera and a sound monitor. So far, zoo officials…