Vesper sparrow (Pooecetes gramineus affinis) COSEWIC assessment and status report: chapter 8

Limiting Factors and Threats

Lack of suitable breeding habitat is the primary limiting factor for Coastal Vesper Sparrow populations. It is assumed that, prior to European settlement, sparsely vegetated Garry oak and related ecosystems or other burnt areas would have been the key open habitats used by this species. The clearing of land for farming and other human uses may have increased the amount of suitable habitat available on the coast in the early 20th century. During the last few decades significant amounts of farmlands and other open areas including Garry oak ecosystems have been converted to industrial, commercial and residential developments, or more intensive forms of agriculture (Campbell et al. 2001). Therefore, it seems probable that availability of suitable habitat has subsequently declined (i.e., estimated 95% reduction: Fuchs 2001). The major cause of habitat loss for this species is intensification of agricultural practices and urbanization.

Agricultural practices that involve mechanical procedures (e.g., tilling, mowing) during the nesting season can destroy nests, having an obvious impact on reproductive success of ground-nesting birds. Modern crop “improvements” of more rapid growth with earlier and more frequent harvest exacerbates the risk to ground-nesting birds. When greater industrialization of agriculture involves the expansion of field size, the subsequent “clean farming” practices of removal of shrubby fencerows eliminates important habitat structural features, reducing the suitability of habitat (Rodenhouse et al. 1993; Sauer et al. 2004). Intensive grazing, where animals are concentrated in small enclosures, reduces the suitability of the habitat and when nesting is attempted in these situations, there is an increased likelihood of nests being trampled (Bock et al. 1993).

Urbanization permanently removes parts of the land base in the footprint required for buildings, roads and other infrastructure. Most of the remaining area (e.g., backyards or city gardens) is also dramatically altered such that it has little to no habitat suitability for Vesper Sparrows (Jones and Bock 2002). Although some habitat within or adjacent to urban areas may appear suitable, Vesper Sparrows tend to occur in lower density in these areas than further away from the urban edges. This indicates that there are factors such as increased disturbance or increased predation that adversely impact the habitat’s suitability (Bock et. al. 1999). On southeastern Vancouver Island, regional parks with potential habitat are often heavily frequented by visitors, which may be detrimental to this species. Within urban regions, the high concentration of domestic and feral cats also poses a threat to this species (George 1974; Cooper 1993; Coleman et al. undated).

Because Coastal Vesper Sparrows currently occur in very small numbers at a single site, the Canadian population is particularly vulnerable to extirpation. A single event (e.g., an infrastructure development project on the airport grounds), could eliminate the entire population.

Page details

Date modified: