BEIJING—A herd of 15 wild elephants that walked 500 kilometers (300 miles) from a nature reserve in China’s mountain southwest were approaching the major city of Kunming on Wednesday as authorities rushed to try to keep them out of populated areas. Chinese wildlife authorities say they don’t know why the herd left a nature reserve last year near the city of Pu’er, a region known for tea cultivation. The group was 16 animals, but the government says two returned home and a baby was born during the walk. Authorities have blocked traffic on roads while the elephants crossed and were setting up barriers and using food as bait to try to keep them away from Kunming and other populated areas. On Wednesday, the herd was in Yuxi, about 20 kilometers (12 miles) from Kunming, a city of 7 million people, the state-run Xinhua news agency said. It said images taken …