In this Jan. 4, 2015 photo, Monarch butterflies climb up a twig, at the Piedra Herrada sanctuary, near Valle del Bravo, Mexico. The population of the butterfly, that migrates thousands of miles each year from winter nesting grounds in Mexico, has been shrinking partly because farmers are growing more herbicide-resistant crops that have stripped millions of acres of milkweed they depend on to nourish them along their route. (AP Photo/Rebecca Blackwell)