Image Credit: Tom Benson
Caterpillar Hosts: Willows including black willow (Salix nigra), weeping willow (S. babylonica), and silky willow (S. sericea); also American elm (Ulmus americana), cottonwood (Populus deltoides), aspen (P. tremuloides), paper birch (Betula papyrifera), and hackberry (Celtis occidentalis). Older caterpillars wander about and may be found on plants that they do not eat.
Adult Food: Mourning Cloaks prefer tree sap, especially that of oaks. They walk down the trunk to the sap and feed head downward. They will also feed on rotting fruit, and only occasionally on flower nectar.
Habitat: Because Mourning Cloaks roam and migrate, they are found almost anywhere that host plants occur including woods, openings, parks, and suburbs; and especially in riparian areas. Less common in Portland area.
http://www.butterfliesandmoths.org/species/Nymphalis-antiopa