Real estate borrowers are leading the rally in U.S. corporate bonds as investors add to bets property companies will weather an increase in commercial mortgage defaults. Bonds sold by real-estate investment trusts, shopping-mall owners and office landlords have gained 3.27 percent this month, exceeding 3.18 percent for all of the fourth quarter, and BBB rated commercial mortgage bonds returned 3.59 percent, according to Bank of America Merrill Lynch indexes.