Introduction

The Western Flycatcher complex is part of the Empidonax genus, in the family Tyrannidae. The American Ornithologists’ Union (AOU 1989) split the complex into two species, the Cordilleran Flycatcher (Empidonax occidentalis) and the Pacific-slope Flycatcher (E. difficilis). Differences in vocalization, size, colour and gene frequencies were the basis for this split (Johnson 1980, Johnson and Marten 1988). Johnson and Marten (1988) proposed a split between the lineage leading to E. d. hellmayri (Cordilleran) and E. d. difficilis (Pacific-slope) at about 250,000 years before present. The two species are visually identical but Johnson (1994) states that they can be distinguished in the field by the advertising song of the males. He describes the pitch changes in the last syllable of the Cordilleran Flycatcher song as a "high-low" sequence of notes while the Pacific-slope shows the reverse (low-high) pattern (Johnson 1994).

 

Figure 1. Distribution of Western Flycatcher complex (adapted from Johnson 1980)

The Western Flycatcher complex breeds from southwestern Alaska south to southwestern California and from southwestern Alberta, through Wyoming and South Dakota to Arizona and northern California (DeGraaf and Rappole 1995). The distribution of the Cordilleran and Pacific-slope forms based on Johnson (1980) is shown in Figure 1. Subsequent research has demonstrated the presence of the Western Flycatcher complex throughout southern British Columbia (Campbell et al. 1997, Sauer et al. 1997, Cannings and Hunn, unpubl.). The Cascade Mountains and Sierra Nevada Mountains separate the coastal Pacific-slope from the interior Cordilleran forms. Both species winter in northern Mexico.

In Alberta, the "Western Flycatcher" is closely associated with slopes, cliffs and canyons especially near creeks and rivers (McGillivray and Semenchuk 1998, Doug Leighton, pers. comm., Fig. 2). Sites with these features tend to be moist and shadier than the surrounding area (Campbell et al. 1997). Johnson (1994) comments that "E. difficilis (the "Western Flycatcher") is invariably associated with water courses and thus openings in the timber … moist canyon bottoms offer the airways under the canopy … for foraging and the sites for nests along mossy stream banks … coolness, shade and nest-sites are indispensable requisites whenever continental populations nest successfully." Like all flycatchers, the "Western" eats mainly flying invertebrates (Bent 1942).

 

 

Figure 2. "Western Flycatcher" site along the Livingstone River.

Johnson (1980), Johnson and Marten (1988) and McGillivray and Semenchuk (1998) assert that the Cordilleran, not the Pacific-slope is the form found in Alberta. By contrast, the recent National Geographic Field Guide to the Birds of North America (Dickinson 1999) indicates the Pacific-slope Flycatcher as the Alberta form. Support for the presence of the Pacific-slope Flycatcher in Alberta is based on a 1993 manuscript (Richard Cannings and Eugene Hunn unpubl.) which argues that vocalizations of "Western Flycatchers" in the interior of British Columbia and the Alberta Rocky Mountains identify these birds as Pacific-slope forms.

Due to the subtle differences between the Cordilleran and Pacific-slope Flycatchers and the uncertainty as to which species breeds in Alberta, we will refer to all birds within the province as "Western Flycatchers". When referring to accounts of species outside Alberta, we will use the name the author uses (Western Flycatcher, Cordilleran Flycatcher, and Pacific-slope Flycatcher).

Historically Empidonax flycatchers have been a challenge to document and study. The similarity in plumage within the genus makes identification difficult. The specific and often uncommon habitat preferred by the "Western Flycatcher" in Alberta presents some special problems, too. Riparian habitat is challenging to census because access by road or trail is often limited and moving water is noisy making detection of singing males difficult. Finally, this species can easily be missed when areas are censused because its arrival on breeding territories may be later than the date of most spring surveys. In the southern interior of British Columbia, the Western Flycatcher arrives in early June (Campbell et al. 1997). Arrival in Alberta is at least this late (Pinel et al. 1993, Leighton, pers. comm.), particularly at higher elevations.

There are relatively few documented records of "Western Flycatchers" in Alberta despite considerable anecdotal evidence. The first record for the province was in 1954 (Salt and Salt 1976). Holroyd and Van Tighem (1983) describe the Western Flycatcher as a rare summer resident with less than 10 records for Banff and Jasper National Parks. The Atlas of Breeding Birds of Alberta (Semenchuk 1992) showed only nine confirmed breeding records for its five year atlassing period. The Status of Alberta Wildlife (Alberta Wildlife Management Division 1996) listed the Cordilleran Flycatcher as "Yellow B" indicating "…concerns related to naturally low populations, limited provincial distributions or demographic/life history features that make them vulnerable to human-related changes to the environment."

The low number of confirmed records for the "Western Flycatcher" may be more a statement of the challenge of censusing this bird than its true rarity. We undertook a survey of the mountains and foothills in Alberta that provide potentially suitable habitat for these species. We had two objectives. First, we wanted to determine the true range of this species in the province. Second, we wanted to quantify the habitat of the flycatcher to assess the risk that habitat loss poses for the species.

Go to Materials and methods