ATLANTA — A giant panda named Lun Lun gave birth Monday at Zoo Atlanta to two tiny cubs, the first twin pandas born in the United States since 1987, zoo spokeswoman Keisha Hines announced. The 15-year-old panda went into labor Monday afternoon and gave birth to the first cub at 6:21 p.m. EDT and the second two minutes later. Hines said zookeepers who had been anticipating only one cub based on a recent ultrasound were surprised by the first-ever twin panda births at Zoo Atlanta.