Status Report for the Oregon Vesper Sparrow

The Oregon Vesper Sparrow (Pooecetes gramineus affinus), a subspecies of the widespread Vesper Sparrow, had an historical breeding range of southwestern British Columbia, western Washington, western Oregon and extreme northwestern California. Historical accounts suggest it was locally uncommon to abundant in the disjunct distribution of grassland and savannah habitat in western Washington. It has experienced range-wide population declines and range contractions, and many local extirpations. In Washington, this includes 20th century extirpations from Vashon Island of the north Puget lowlands, and the Dungeness area of the Olympic Peninsula; and extirpation from San Juan Island appears likely. Approximately 90% of the population occurs in the south Puget lowlands, predominantly on Joint Base Lewis-McChord. The current range-wide population estimate is <3,000 birds with approximately 300 of those occurring in Washington. The Oregon Vesper Sparrow was petitioned for listing under the Federal Endangered Species Act in October 2017. In June 2018, the U. S. Fish and Wildlife Service determined that it may be warranted for listing and initiated a 12-month status review. It has been a candidate for state listing in Washington since 1998.

Oregon Vesper Sparrows are present in western Washington primarily from early April through late September, and they migrate to winter in central and southern California. They feed on a wide variety of insects during the breeding season supplemented with seeds during migration and winter. They are a ground-nesting bird that matures to breed in one year and maintains monogamous pair bonds. They will renest after failures and can produce two broods in one year. They exhibit relatively high annual site fidelity to nesting areas and defend small territories during the breeding season.

Oregon Vesper Sparrow breeding habitats in Washington include herbaceous-dominated, open upland landscapes such as prairie and savannahs, pastures, airfields, Christmas tree farms, and vegetated dredged material sites. Within these habitat types, desired habitat conditions can be characterized as moderately short and structurally diverse grass and forb cover with some patchy bare ground and sparsely vegetated areas, low to moderate shrub or tall forb cover, and low tree cover.

Historical population declines were likely the consequence of the extensive loss of prairie and savannah habitats. Loss and degradation of habitat continues to be a factor in both breeding and wintering areas, but current threats likely include demographic and genetic issues associated with small and isolated subpopulations, and predation and disturbances during the nesting season. Another potential threat is exposure to pesticide-treated seeds, especially in agricultural habitats during migration and winter.

Restoration of prairie and savannah habitats, particularly for the endangered Streaked Horned Lark and other prairie butterfly and plant species, has been a conservation emphasis in Washington in the last 20 years. This has the potential to benefit Oregon Vesper Sparrow populations if specific habitat conditions are created or maintained. However, where complete removal of trees and shrubs has occurred, Oregon Vesper Sparrow populations may have been negatively affected.

Given the small population size, numerous local extirpations or near-extirpations, the variety of nesting disturbance factors that may negatively affect them, and the lack of species-specific regulatory protections, it is recommended that the Oregon Vesper Sparrow be classified as an endangered species in Washington.