Brown-headed Cowbird (Molothrus ater)
Limiting factors are considered to be those conditions which degrade habitat suitability, reduce survivorship of young or adults, or decrease nesting success of adults once they are established to a site (Prescott 1997).
Livestock grazing has been found to be detrimental to the Willow Flycatcher because of the associated cowbird parasitism, and destruction of vegetation (Taylor 1986, Saab et al. 1995). Grazing in willow or brushy habitat often removes the vegetation at the level which the Willow Flycatcher nests (Taylor and Littlefield 1986). Loss of habitat through cattle grazing does not seem to be of significant concern in Alberta. Few sites supporting Willow Flycatchers are currently found in association with cattle. In fact, most of these sites would not be suitable for grazing. Willow Flycatcher habitat in Alberta is usually too moist and the willows too dense to be accessible for grazing. Some of the sites on the eastern edge of the Willow's range in Alberta may be suitable for grazing which might limit eastern expansion of the Willow Flycatcher.
Many locations that have historically had Willow Flycatcher records did not yield any sightings during our 1999 survey. These were mostly grazed dry-upland sites in the southern foothills. These sites support grazing which could explain the lack of recent Willow sightings, or a combination of grazing and cowbird parasitism may limit productivity.
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Brown-headed Cowbird (Molothrus ater)
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Brown-headed Cowbird nest parasitism is of major concern for Willow Flycatcher populations. In the southwestern United States, nest parasitism usually leads to complete nest failure (Sogge et al. 1997). Campbell et al. (1997) reported 44% of Willow Flycatcher nests in interior British Columbia were parasitized by Brown-headed Cowbirds. Cowbirds are common in the dry, grazed upland sites in Alberta where Willows were historically reported. We recorded cowbirds at a significant number of ungrazed, wet lowland sites indicating the potential for nest parasitism. Cowbirds benefit from the insects disturbed by grazing cattle and are often found in direct association with livestock. Some studies have found that female cowbirds will commute up to 7 km from its feeding sites to breeding sites (Rothstein et al. 1984, Thompson 1994). Due to the high songbird density and diversity in riparian or wetland habitats, they are prime locations for Brown-headed Cowbirds. Several studies in the United States have looked at the effect cowbirds have in these habitats (e.g.) Schweitzer et al. (1998) in New Mexico, Staab and Morrison (1999) in Arizona, and Tewksbury et al. (1999) in Montana.
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Figure 12. Willow Flycatcher site in
the Bow Valley. Note the the rail line bisecting the habitat.
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The Willow Flycatcher typically occupies habitats not traditionally sought after for recreational or commercial developments. In Alberta, the Willow Flycatcher can be found in shrubby wetlands, road and rail ditches and brush around lake margins (Fig.12 and 13). Wet, willow lowlands do not lend well to development, logging or any other foreseeable modification. The "cleaning" of willows and shrubs from lake edges could reduce available habitat but the greatest risk is lack of knowledge about the importance of the otherwise insignificant "waste" areas to this species. Road expansion, wetland drainage, or cleaning of rail allowances could easily eliminate a significant number of Willow Flycatcher sites.
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| Figure 13. Willow Flycatcher habitat along Highway 1A in the Bow Valley. |
On the assumption that habitat loss from grazing and cowbird parasitism may be restricting Willow Flycatchers to wet habitats, they can expect to encounter Alder Flycatchers in these habitats (Fig.14). As noted earlier, the outcome of interactions between these two species is unpredictable. We found Willows and Alders together in 10% of the sites and also found only Alders in sites that appeared suitable for Willows. Prescott (1986) noted that both species excluded each other from their territories. If suitable habitat is limited, and the Willow and Alder are competing for this space, historical evidence suggests that one species tends to eliminate the other. In eastern Canada, the Willow Flycatcher is thought to be forcing Alders out of their habitat (Prescott 1987) while studies in British Columbia (Campbell et al. 1997) and Wisconsin (Robbins 1974) show the Alder forcing out the Willow. Because Alder Flycatchers are far more common in Alberta, should they move in substantial numbers into current Willow Flycatcher habitat, the Willows may be at risk.
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Figure 14. Alder Flycatcher (Empidonax
alnorum)
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