Introduction

Editor's Note: This article covers two species: Pacific-slope and Cordilleran flycatchers. As what follows makes clear, these species have been considered as one in the past, so BNA initially treated them in a single account. Future revisions will provide separate accounts for each species.

Aptly named difficilis. Difficult. Empidonax difficilis. The difficult gnat king. Difficult indeed, and doubly so now. The name “Empidonax difficilis” has had long association with the bird known as the Western Flycatcher. Recognition that 2 species existed under the guise of the Western Flycatcher followed a comprehensive systematic study of geographic variation characterizing differences in size and color, vocalizations, genetics, and ecology between coastal and interior populations. The 2 species divide 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. This division is not a final simple solution for “Western Flycatcher.” More study is needed of interactions between Pacific-slope and Cordilleran flycatchers where their ranges meet in the interior Northwest. Also, populations breeding on the Channel Is. off s. California, currently treated as the Pacific-slope Flycatcher subspecies E. d. insulicola, may actually be best treated as a distinct species (see Systematics, below).

Many aspects of the biology of these species appear similar and general statements about either can probably refer to the other as well -- or even, in a more general sense, to other Empidonax flycatchers (e.g., McCabe 1991; Sedgwick 1993, 1994, 2000; Briskie 1994; Lowther 1999). The few published studies of Western Flycatcher nesting biology refer to the coastal Pacific-slope Flycatcher. Three major references of this species have included detailed observations of 7 nests of 5 pairs in Monterey County, California (Davis et al. 1963); detailed observations of the entire nesting cycle of 3 pairs in Humboldt County, California (Sakai 1988); and studies of vocalization behavior and nesting of 7 males on Queen Charlotte Islands, British Columbia (Ainsley 1992). Few observations have been made of Cordilleran Flycatcher biology. Systematics and geographic variation (including morphology, vocalizations, genetics, and habitat) of both species were detailed by Johnson (1980, 1994b) and Johnson and Marten (1988). Molt occurs primarily on the wintering grounds (see Johnson 1974).

In this account, some observations of Western Flycatcher from pre-1989 literature can be allocated to either Pacific-slope or Cordilleran flycatchers based on geography; use of name Western Flycatcher indicates uncertain identity.