To date, investigation of STHL breeding ecology in Oregon has largely been confined to the southern half of the Willamette Valley in the core of the subspecies’ range. Because STHL breeding habitat (open agricultural land) is quite similar from southern Lane County to northern Yamhill County, information on breeding ecology from Benton, Linn, Lane, and Polk Counties will likely be applicable in more northern agricultural counties in which STHL breeding ecology has yet to be investigated.
STHL breeding ecology in Multnomah County, however, may be distinct from that in the southern valley, as both the suburban/industrial landscape setting and at least some habitat types (dredge material deposition sites) differ. Details of STHL breeding ecology in this county may be of special import, as the small breeding population there appears to be the southernmost of a string of disjunct populations to the north of the continuously distributed mid-/southern Willamette Valley population. The nearest robust population to the south appears to be some 55 km. distant in northern Yamhill County (and may be considerably farther south than this; the area has not been formally surveyed), and the nearest known populations to the north are White’s Island (71 km.) in Oregon and the Olympia Airport (149 km.) in Washington.
What follows details the results of fieldwork conducted in Multnomah County, Oregon during STHL breeding season 2010. This work was designed to delineate STHL breeding distribution and abundance at the county’s two known breeding sites; to identify habitat- and locality-specific values of reproductive success; to identify variables, if any, that are associated with reproductive success; and to identify sources of reproductive failure. 5