Mourning Cloaks are consistently the first butterflies to appear in March each year. While they prefer to feed on tree sap, oaks are a favorite, or rotting fruit, they have to make due with sucking nectar from the blossoms of our early blooming plum tree. The Mourning Cloak caterpillars like to feed on a variety of tree leaves including elms and cottonwoods, which we have plenty of. Adults that appear this early in the season have hibernated over the winter.