The Streaked Horned Lark (Eremophila alpestris strigata) is a critically endangered subspecies which breeds on prairie remnants in Washington and Oregon. Dramatic losses in grassland habitat have pushed the lowland Puget populations to the brink of extinction, with projected population losses at 40% a year. In order to investigate potential mechanisms driving this decline, I conducted a case study of Streaked Horned Larks at 13th Division Prairie, Fort Lewis, Washington over a two year period, 2007 and 2009. I analyzed nesting data of all species comprising the grassland ground nesting guild, and compared Streaked Horned Lark fecundity with those of the larger guild to determine if the breeding site itself is a sink, or if low fecundity is specific to Larks. I compared fecundity in two separate groups: (1) Larks vs. the ground nesting guild and (2) Larks vs. Savannah Sparrows (Passerculus sandwichensis). In these comparisons, Streaked Horned Larks had significantly lower values in all measures of reproductive success when compared to both the guild and Savannah Sparrows. Furthermore, the Streaked Horned Lark’s low egg hatching rate of 44% suggests that inbreeding depression may be playing a role in the decline of Larks at 13th Division Prairie. Although analyses of nest site habitat variables confirmed that Streaked Horned Larks have unique nesting preferences, cross-year, interspecific comparisons of vital rates and nest site characteristics did not indicate site-wide environmental causes driving Streaked Horned Lark declines. Since these findings are based on a case study of a single breeding site, I recommend further monitoring of this site and other remaining breeding sites, with emphasis on potential inbreeding depression.