Aptly named difficilis. Difficult. The name “Empidonax difficilis” has had long association with the bird known as the Western Flycatcher, but this entity is actually 2 species whose breeding distribution is divided roughly along the Cascades and n. Sierra Nevada into the Pacific-slope Flycatcher west of the divide, retaining the name Empidonax difficilis, and the Cordilleran Flycatcher, taking the name Empidonax occidentalis, found to the east to the Rocky Mountains and south in mountains throughout most of Mexico. The species' biology is undoubtedly similar to that of other Empidonax species. Molt occurs primarily on the wintering grounds.