Limiting factors are considered those conditions that degrade
habitat suitability, reduce survivorship of young or adults, or decrease nesting success.
Some limiting factors of the Willow Flycatcher include Livestock
Grazing, Cowbird Parasitism, Habitat Loss and Alteration
as well as Competition with Alder Flycatchers.
Grazing in willow or brushy habitat often removes the vegetation at the level that the Willow Flycatcher nests. However, loss of habitat from cattle grazing does not seem to be of significant concern in Alberta. Few sites known to support Willow Flycatchers are associated with cattle grazing and most Willow Flycatcher habitat in Alberta is too moist and the willows too dense to be accessible for grazing. Some of the sites on the eastern edge of the Willow Flycatcher's Alberta range may be suitable for grazing, which might limit the expansion of the
species towards the east. Many locations with historical records of Willow Flycatchers did not yield any sightings in 1999. These were mostly dry, grazed upland sites in the southern foothills. This alone, or a combination of grazing and cowbird parasitism,
may explain the lack of recent Willow Flycatcher sightings in these areas.
Brown-headed Cowbirds benefit from the insects disturbed by grazing cattle and are usually associated with livestock. Brown-headed Cowbird nest parasitism is a major limiting factor for Willow Flycatcher populations. 44% of Willow Flycatcher nests in interior British Columbia were parasitized by Brown-headed Cowbirds. In Alberta, cowbirds are common in the dry, grazed upland sites where Willow Flycatchers were historically reported, but no longer occur. The incidence of cowbird parasitism decreases with the increase of volume of vegetation below and surrounding the potential host's nest and the presence of medium to large size trees within the habitat.
Habitat Loss and
Alteration: In Alberta, the Willow Flycatcher typically occupies habitats undesirable for recreational or commercial developments. Wet, willow lowlands do not lend well to development, logging or any other man-made modification. The removal of willows and shrubs from lake edges could reduce available habitat, but the greatest risk is lack of knowledge about the importance of
"waste" areas to this species. Road expansion, wetland drainage, or cleaning of rail allowances could also eliminate a significant number of Willow Flycatcher sites.
Competition with Alder
Flycatchers: The outcome of interactions between Alder and Willow Flycatchers is unpredictable. It has been noted that both species excluded each other from their territories. In eastern Canada, the Willow Flycatcher is thought to be forcing the Alder Flycatcher out of its habitat while studies in British Columbia and Wisconsin show the opposite situation. Alder Flycatchers are far more common in Alberta therefore Willow Flycatchers may be at risk from competition with Alder Flycatchers.
Reprinted from Alberta Wildlife Status Report No. 29
with permission from Alberta
Sustainable Resource Development.