In this Jan. 4, 2015 photo, rabbles of Monarch butterflies hang from tree branches, in the Piedra Herrada sanctuary, near Valle de Bravo, Mexico. More butterflies appear to have made the long flight from the U.S. and Canada to their winter nesting ground in western Mexico, raising hopes after their number dropped to a record low last year. But experts still fear that unusual cold temperatures will threaten the orange and black insects. (AP Photo/Rebecca Blackwell)