MADRID, Sept 6 (Reuters) - Twin giant panda cubs were born at Madrid"s zoo on Monday in a boost to conservation efforts for the vulnerable species, whose numbers have been rebounding thanks to an international captive breeding programme. The pair, whose sex is yet to be determined, are the fifth and sixth cubs of Madrid"s female panda Hua Zui Ba and her partner Bing Xing, the zoo said. After four hours of labour, the first cub was born around 8:30 a.m., while the second followed four hours later in what the zoo described as a "peaceful" birth. Two technicians from China"s Chengdu panda breeding base will assist local veterinarians in checking the health of the pink, hairless newborns, who will be totally dependent on their mother for the first four months of their life. In July, Chinese conservationists announced they no longer considered pandas to be an endangered species, upgrading their status a notch to vulnerable. The International Union for Conservation of Nature made a similar change to its classification in 2016.